6061 does not bend well, rather it is prone to cracking.
There are other aluminum alloys that are better suited to bending. One that I am familiar with is 5052.
I don't think that it will be easy to bend 0.250" aluminum by hand, however if you take into account the lever action of the fret bar, there would potentially be more force available. You can find 5052 aluminum at a Lowes store if you want to do some experiments (I have bought 0.125" thick 5052 bar from Lowes, but I'm not sure whether or not they carry it in 0.250" thickness).