i personaly wouldnt drill a hole to hide wires , i know they do it all the time on show bikes but on an every day daily commuter no way ! imo i also do not claim to be an expert , but if my swing arm devolped a crack , i wouldnt drill and weld it either . i know the weld is stronger than the metal ! but if not done correctly it could comprimise the tensile strength of the surrounding area ? so i myself would have to pay to have it welded $$ , so i would opt to just replace it with a used non cracked / drilled one. just my opinion .
i have had lots of stuff welded for my job by certified proffesional welders 'snow plow frames " and yes the weld is strong and should never brake , but on more than one occation i have developed tears or breaks close to the weld ? what would cause that ? this is what scares me about welding on a bike . i wouldnt do it , but its my opinion , and you know what they say about those
so to each his own
and good luck ! very clean install looks great ! could the tag holder be mounted on the caliper side ? cause then you could run a tiny black wire along the brake line for the rear caliper ,for power and use the bracket itself for a ground these are led lights ? so the wire could be realy tiny almost invisable ? even with a plate light ,you might still not be legal ? maybe in ga but not ny . though it is rarley enforced