It's a very dated/formal use of the subjunctive, and as time goes by I expect more and more texts will be revised to ...that mourns in lonely exile here until the Son of God appears. Another problem is that I suspect the vast majority of "well-known" sentences starting with an imperative are likely to be addressed to some kind of god who might answer prayers.

@Fumble Fingers I grant you that what I would call the subjunctive of indefinite future is dated, maybe even obsolete, but I think it will stay in the sentence, because it is needed for the rhyme. Also any choir director will tell you that it is hard to get a final s sound in sync. So the person in charge is likely to prefer "appear."

You'd do better not to mention the subjunctive mood in an English class. Modern English has no subjunctive mood. It does have a couple of rare constructions that some people mistakenly called "subjunctive" because they mean something similar to a couple of Latin subjunctive mood usages. But it's about as important as learning how to inflect a verb with the pronoun thou (thou art, thou hast, thou canst, thou must, thou knowest, ...).

'appear' is unlikely to be changed because of the rhyme.

@John Lawler My previous comment was addressed to you, not DJClayworth, with whom I quite agree.

I find it useful to refer to that mood as "a mood which may conveniently be called the subjunctive". But then I do have an active interest in English liturgy, so knowing the difference between hast and hath is important.

I don't know if it's well-known, but the closest thing I could think of was something like this: "Tell them, as you make plans, that it's important that they be on time".

"Subjunctive mood" was the phrase; English doesn't have such a mood. (BTW, I started my career teaching ESL and founding an English Language Institute). As I said, there are a couple of minor constructions that are called "subjunctive" by some. But not by others -- Huddleston and Pullum, for instance, use it for mandative that-clauses with untensed verbs (It's important that you be there), but not for the irrealis past construction (if wishes were horses), which is completely unrelated.

I think you'll get a long argument about definitions if you ask linguists about "subjunctive mood" in English. And, by the way, what do you mean by "linguists"? I have my own definition, shared by other linguists, but it might well not be yours. Bas Aarts is a linguist, but notice that he doesn't say there's a subjunctive mood in English, and he limits "subjunctive clause" to untensed that-clauses governed by a small class of predicates. That makes it a descriptive category rather than a grammatical one, like calling something a purpose infinitive or a counterfactual conditional clause.

I'm not from the south, but I know that Bless your heart is an imperative involving the relative placement of other parts of the body.

If you're not teaching the example as dogma, why is it problematic in English class?

There was an art teacher teaching calligraphy, who had her students draw some Arabic calligraphy that turned out to mean something religious, like God is great. She got in trouble for that, and the sentence at hand has a great deal of religion in it. I'm not saying it's sensible to teach art or English without ever mentioning the world's great religions. But a secular sentence would be safer.

@deadrat I do not believe Bless your heart is an imperative. I think you are wishing for God to bless. Similarly, "God damn it" expresses a wish for God to damn it, not a command to God to damn it.

@Airymouse I take your point. I should have said, "If you're not teaching in some godforsaken hellhole of ignorance near the West Virginia state line -- and why would you waste your time doing something so futile? -- and your example is from the majority faith, why is it problematic?"

@Airymouse 1) Don't confuse the semantic issue of a command with the imperative issue of syntax. God damn it is an imperative because of the verb form. A sentence in the indicative mood would be God damns it. In this case, it's also a command since it means God [,you,] damn it. 2) My comment is a joke, if, perhaps, not a particularly funny one. It might be easer to tell if I weren't too old to use emoticons, but I'm not and I don't. I'm saying Bless your heart really means Pull your head out of your ass.

@deadrat I didn't get your joke, but I am impressed that you can figure you that for me the West Virginia state line is about a forty-five minute drive away..

Here's the start of a famous secular sentence: To be or not to be, that is the question: ..." This initial phrase alone hits 2 out of 3, missing only the imperative mood.

@Airymouse That's all right. You're in good company. I'm mostly here to amuse myself. I'm not clairvoyant. The calligraphy brouhaha took place in Augusta County, VA, which shares a boundary with WV. I'm either sorry you live where you do, or I'm sorry that I disparaged where you live, whichever is less offensive.

@Lawrence No, that's only the imperative. To be or not to be is not subjunctive, but infinitive.

@deadrat Morphologically, the present subjunctive is identical to the infinitive and the imperative, so you can't really say categorically whether God damn it in Modern English has an actual imperative (extended from the second to the third person) or a subjunctive (functioning as the semantic imperative for the third person). It could be either, and there are arguments for both. I personally tend to favour the subjunctive analysis for historical reasons.

@JanusBahsJacquet Does this claim the phrase is in the subjunctive? (Page 83, towards the end of the second paragraph.)

@Lawrence No, it doesn’t seem to. If you look at the bottom of page 78 in that book, it gives the context in which to be or not to be occurs: as an English more-or-less equivalent to Guillaume’s notions in French of chances d’être (‘possibility to be’) and chances de ne pas être (‘possibility not to be’), the Occurence and Alternative to Occurrence that Guillaume and Dreer use as (one) basis for defining the difference between the indicative and the subjunctive. So he’s using to be or not to be to describe what the subjunctive is, not as an example of a subjunctive.

@JohnD You should post that as an answer—it is a very good example that shows that it is entirely possible to make such a sentence, even without sounding strange or contrived.