Had not really thought of wanting to do what your asking before, so I did some hunting around. Unfortunately, I struck out on coming up with a way to do it without uninstalling the apps that you don't want to show up.
The way that list works is that when an app installs that wants to be a part of the list, it essentially "registers" itself with the Android OS to make itself available for sharing. I can't come up with any way within the normal Android GUI to edit those registrations. I poked around in the underlying OS for a while to see if I could find a config file of some sort that could be edited (assuming you have root), but didn't find anything. It might be worth posting this type of thing over at xda-developers forum to see if one of the hackers over there can figure something out.