Can you add a trailing space to every single line of the code and release it on github as "GPLv2 and later" (hey, it's a patch), thus getting rid of the whole "GPLv2 only" clause?

No, a patch that doesn't completely replace existing code can not alter it's copyright. As long as you can insure that your GPLv2 code is in the project and not easily replaced, you control the project's license.