I've seen it done a few times, it can work, but I doubt you'd save much. I drilled 4mm holes in a Monty TI rim, 168 of them actually, and it saved 18g. From looking at that rim, you could run a rim brake on it. I saved 50g by switching from Tarty Griffles to TrialTech Foam. On my current front rim on my stock, I drilled 256 7mm holes in it, they're really close together, they're huge and it saved 60g. You could never run a rim brake on there though, there's more hole than sidewall left!