Yes, yes, yes. Remember that the 1/4" hole is going to remove closer to 7/16" of metal by the time you thread it. Cast aluminum risers are pretty rare birds since they are exceptionally heavy. If it is cast I'm betting it's magnesium. If you don't know what you're doing when you go to thread it, take it to a machine shop. The last thing you want to do is bust off a tap that big in your riser and aluminum and magnesium tend to grab taps pretty hard if you don't go slow and clear often. That's one reason why some of the manufacturers insert a threaded steel bushing rather than threading the riser itself.

Personally I'd get one that either side mounts or mounts to an adapter in the front stabilizer hole. I'd leave the riser alone. (Unless you've got a "friend" that you don't mind getting rid of that will test shoot it for you a few times afterwards.)