It seems like I've heard that some of the free float rail systems out there will flex when pressure is put on them by the sling. Obviously, one way to deal with this would be to mount the sight on the gas block, but it would be nice to have the additional sight radius possible with putting the front iron sight out on the end of a long rail setup. Does anyone here have experience with what does and doesn't work for appleseed?

Aero Precision M4E1.  Yeah, you'll have to go with their M4E1 upper to use it.  It'll be worth it.

Taunt not tight. Id be surprised if they flexed. Especially if the direction isn't perpendicular. And your hand is up against the swivel.

A monolithic upper would be your best bet, followed closely by the M4E1 (which is also MUCH cheaper). They still flex if you put a ton of pressure on it, though.

Thanks for the info, guys. :-)