ORIGINAL: TFOX
Drilling an extra hole in a cast magnesium riser is going to really put a weak spot in the riser.I am a tool maker by trade and I probably wouldn't do it.
I have seen magnesium risers break in half without extra holes being added.
With that being said,you should have a competent machine shop look at it and see if the area you want to add a hole to would be thick enough to handle the stress after the hole would be installed.
Good advice.
I've drilled holes in customers aluminum bows after making them sign a waver. Seriously. I hate magnesium riser bows... make me nervous. I've seen too many with warped risers... and sadly almost all of them were due to archery "pro's" incorrectly pressing them.
I've built a few of those STS/vibe-killer whatever arms into bows directly. I did one last week on a PSE whitetail obcession or whatever that super short 8" brace bow they made a couple years ago was. Took a Cobra cable slide bar that was threaded to the same pitch as a stabalizer... took a standard tap for the same pitch (every archery shop will have one) and drilled a 17/64 hole as listed on the tap.... a little oil... tap handle... hacksaw off the extra bar... round off the edges on a grinder.... rubber cap on.... bow is 3x quieter.