OK, so I tried to RF the Foonf on the way home and no dice - couldn't even jam it down to even get to the point of trying the belt path. I think based on where the buckle will end up that it won't work anyway, even if you had the room to do it. It might fit w/o the ARB, but I don't think that's allowed, right?
Anyway, it worked out for us with the Radian and the Foonf because the Foonf has a couple of things going for it:
1) Compatibility with the Go-Go Babyz wheels - very, very solid install and was a great place for the 11mo to ride around the airport, plus makes the weight of the Foonf almost a non-issue since it is narrow enough to fit down the aisle.
2) Works great FFing, although it does sit so high it makes the tray table unusable.
So, it was a bit of work since the 11mo rode in the Foonf until we got onboard, then I got the Radian installed RF'd (with AA, and then only barely fit), then put the 3.5yo FF'd in the Foonf.
Overall, I think if we were frequent flyers and/or we had a destination that required multiple connections, I'd probably just get some smaller, lighter seats for travel. Not sure what those would be exactly without compromising what I'd want at the destination, but we travel so infrequently that I'll just suck it up and keep using this setup for now ...