Well evidently it IS possible to get away with it since 3 or 4 have said they did it to theirs.
You need to think about the type of wood the particular part of the riser you want to drill is made of, how much wood is there, how close to the fades you want to drill etc. I bet you could get away with it but my concern would be long term.
I had a Herters recurve that was drilled and tapped for an insert from the back side right at the throat of the grip. Must have lasted a lot of years as it was an old bow but it broke completely in two there during shipping to me when I bought it. I realize getting broke during shipping is different than being broken during use but still, the insert was very obviously the weak point and I have to wonder if the bow would have made it in one piece if it hadn't been drilled? No matter how careful we are with our bows, hunting brings unexpected opportunities to damage our gear. Ever fallen while carrying your bow? Ever fall on it, drop it from the tree or have to throw it aside when you start to fall on a steep climb? Just think of the possibilities. Longbows are wicked tough. Why weaken one?
Personally, I wouldn't do it unless you could find a smaller insert.