Okay, did a bit more digging. It's not user rep-based, it's a site setting we need to request. From my read of past Meta threads about it, these options are added only for frequent / high-volume migrations, where mods alone would have trouble keeping up.
In our case, because the migration rate is low, the recommended approach is to flag for moderator attention, describing your proposed migration in the comment, and we can navigate the migration using our mod tools if we have consensus.