Also theming the backside of GMR, with fake facades would be pretty simple.
Some bricks, fake windows, etc
I agree, that would be even better than just removing the billboards. It seems like a reasonable thing to do, since MMRR means the building will be sticking around for a while.
Unfortunately, with no Return On Investment for such a project, it doesn't seem like they'd be motivated to spend the money that would be necessary (likely millions, even for just a few relatively simple facades).
This should have been part of the Toy Story Land budget - and maybe it was, in the early planning stages - but alas...