I guess so a 64-bit MSYS2 shell is enough to create a 64-bit build of Geany. But I didn't test it now and I think I never did before.
I don't remember the previous answer, too :(.
Just to be clear: I personally won't do this as it is too much work, IMO. Better just don't stress your clipboard with 200MB data. Anyone is welcome to maintain the Windows builds and then he/she can also build and provide 64-bit builds.