The topic has indeed come up before and I could certainly consider working to support this eventually.
Actually I should already have a GameMaker: Studio Professional license that I obtained through a Humble Bundle, but I haven't gotten around to try it yet partly because I mostly work on Linux. Thanks for the offer though!
Yes! As soon as there is an issue about this on GitHub anybody can add bounties to it through Bountysource. Also, anybody is free to work towards resolving the issue and claiming the bounty.
Recently I've placed some bounties using money received from supporters, as an incentive for people to work on Tiled. This was quite successful with two developers being awarded a bounty.