216 posts?
You've been around here long enough to answer your own question, if you've been paying attention.
12 or 20?
20 is easy, the answer is yes, it can be drilled and tapped, and the 22" youth choke tube barrel is the obvious choice, but you'll have to special-order it, as it isn't listed in the accessory barrel list.
12 ga is a special case. Most 'smiths won't D&T it, but you can have a mount silver-soldered. Some have done it with the T/C steel Weaver rail, or with custom rails.