Having posted the above, I found that the metacell project became an "insurmountable opportunity"... so I did the next round of construction, and left a manageable-sized challenge or two on the conwaylife forum thread:
http://conwaylife.com/forums/viewtopic.php?f=2&t=2865&p=59016#p58541
The metacell mechanism is in working order now. Once the SW corner circuitry is packed into the 512x512 box properly, the only thing left is to wire up inputs and outputs to neighbor metacells -- and optionally add a metapixel area, but that's a fairly simple add-on.

The metacell mechanism is actually complete as of this morning, at least to the point where people might want to look it over and make sure I haven't made any silly design errors:
http://conwaylife.com/forums/viewtopic.php?f=2&t=2865&start=25#p59024
Program the metacell in Golly by adding and subtracting eaters along the labeled diagonal, then test it by sending in a glider for each ON neighbor, on the COUNT input lane. A glider will show up at OUT whenever the metacell is in the ON state, at around T=10000 + 16384N. (I may be able to reduce the period to 8192, but don't want to push my l…