Everything is possible.
Problem lies in, what your asking for is a nullifcation of the pick up rules inside l2server.exe.
And from the looks of your post, you can barely add an item :O - but lest not judge.
Extra problem being, you'd want the condition only active from drops by RB's.
I'd suggest investigating CWorld::PickItem/TryPickItem.
could be as simple as nopping a few compare's.
If this make no sense, you might wanna give up on your request and outsource it - so to speak.