Can I repair this valve stem hole?
I would not fibreglass it - instead I'd remove the black coating 10-15mm around the edges of the hole and just over the "curve of the well". 

Then I'd use a thin piece of aluminium plate and carefully conform it to the bed of the well.  This would be a lot of careful hammering with a ball-peen hammer and perhaps a cold chisel.  You also want the edges napped or relieved to smooth the transition up and down for the rim tape.  No sharp edges !

Once its all fitting in place snugly, I'd drill the valve stem hole in a pillar drill, and carefully chamfer the edges of the hole.  

Weigh the final piece now.

Clean both surfaces and then use a 2 pot epoxy to snot the patch part in place.  Use a bolt temporarily through the valve stem hole to retain it while the epoxy sets, and leave for at least the maximum "hardness" time on the epoxy instructions.

You want to keep it totally below the "terrace" beside the clincher bead hooks, so the repair doesn't interfere with seating of the bead.

Also, you may choose to make a second plate of equal weight to adhere diametrically opposite the valve, to help with balancing the wheel and reduce vibrations at high RPMs.