That's an awfully thin piece of aluminum to try to weld. I would think that the chances of doing more harm than good are fairly high here.
Isn't this completely hidden anyway when the marker is assembled? I would be tempted to just file that little piece of metal away (turning the hole into a slot) and call it a day if it were me.
yes, it would be a problem welding it. you would have to spot it, then clearance the hole for the sear, clearance the valve and clearance the rail. it would be possible to braze it, which would fix it, but brazing is a lost art and it might not be a total fix, i.e. it could break again. it is an extremely thin area to fix, and that i don't believe that even if it wasn't fixed, it shouldn't be a problem.
I wouldn't worry about it as is. Its hidden. If you are worried about it interfering with the on-off at some future time, then preemptively file it into a slot as mentioned previously.
Except for the Automag in front, its usually the man behind the equipment that counts.