I just got done building a gbaHD and was wondering if its legal for runs

Not for the moment but if it does not provide any advantage over console then it can be considered a valid way to run.

For example a lot of testing was done on the SuperNt and after a while it was accepted in some boards; that can happen here with the gbaHD but we need to test it first.

As a verifier I can only check runs that are done on the allowed hardware or software and therefore I can't give a final answer about gbaHD. I suggest you to reach Super Mods through their socials because allowing another console can be complicated if we're not sure about its performance. Umari0 tends to be a little busy but you can reach him out, he will answer back!