"Sharing it would be very difficult for other reasons as well. Its sheer size, roughly 200k lines of source."

That's perfectly valid. Few of us here would be interested in wading through even a few thousand lines of code; certainly not 200,000 lines. In any event, that would far exceed the amount the site would let you post: I think the node limit is 64k characters. However, if you did track it down to some manageable code fragment, and provided that as an SSCCE, you'd be likely to get some positive feedback.

"It's been suggested that I somehow cut back on code until I find the culprit. This does not sound fun."

My suggestion of turning off the warnings was intended as an alternative to actually removing chunks of code. The idea would be to turn off warnings in parts of the code: when the warnings disappeared, that would be code you could investigate further. Obviously, without any knowledge of your code, I've no idea how useful or practical that approach might be.