I’ve decided to punt. I cleared out rails all the way back to 2.2.2 and built the app from scratch again up through the invitations page.
Works perfectly straight out of the box using fbml. I don’t even need to use tunnlr.
All the configuration is copied straight from the apps that didn’t work under 2.3.2 and 2.3.5, and the FB config is untouched.
Seems something in the last half-dozen versions of rails broke something my system needs.
Depending on how determined I feel (I really need to move forward with this app, so I may not bother) I may try installing each significant rails release and rebuild the app until it stops working again: at least then I’d know the most-current version of rails I can use. Since it didn’t work under 2.3.2, I’m guessing the 2.3 series changed something significant.