I needed a very tall (don't have enough styrofoam or rKT), lightweight hat, (not much structure under it so I made a frame out of poster board. Can I cover it in fondant and it won't slide off? has anybody ever done something like this?

Sure. Attach the fondant with vegetable shortening [crisco] and you'll be able to move it around to fit it nicely.  You can use water, but that actually "melts" the underside a bit, leaving less working time, and it can erode thin spots in the fondant.

I've done that with a cardboard cake. Like said above, added shortening and it worked well.

*A cake board, that stood in the back of  the cake. :)