Since beadlocks are common, how big of a deal is it to cut alumium rims in half to make them wider? like if I wanted to make 15x9" Jeep rims from 15x7s?

It can be done. But it's cost prohibitive. you'd pay waaaaay
more to have your rims widened then you would pay for some
aftermarket rims at the width you wanted. figure it would cost
you about 150-200 per rim to widen them.

--Sherpa