Not sure if anything like this has been suggested already, or if there's a quick way to do it already... Perhaps add some sort of little list of "quick references" for map making? Something right below the add/fill/select, where you could drag stuff from the object tree and access them quicker. I know I could basically make a "pallette" by sticking some instances on my map and use "Make Active Object", but something that functioned like a... filtered object tree almost... that was handy on the side there, would be really helpful for map making when your object tree is massive and you really only need a few turfs/objs/mobs at any one given time.
Nice idea. The screen real estate could be a problem, but if there was a way to make it work it'd be a wonderful feature.

I give this idea four acorns.
Pardon the necromancy, but I just wanted to expand on this subject to include "instances" not just items off the object tree if possible. Kind of like BYOND displays in the instance box when you have multiple instances defined for the same type path. Rather than only displaying the instances for a particular type, maybe have another box below it where you can drag instances down into and use for a map that uses a lot of the same group of instances. For now, I have to kind of make a "palette" on my map somewhere, and use Make Active Object to "select" my instances. The instances are especially important for my new material system in Maeva, since I define colors in dmps.

Just a reminder, if it's already been posted, type paths don't seem to show up correctly in the instance editor. My material variables are type paths, and don't show at all. When I try to define them in the instance editor myself I recall getting errors. I'm actually able to edit the dmp file in notepad and accomplish the same thing, however.

Anyways, progress on Maeva is happening again, and that's what's important. A big thank you again to the entire BYOND staff for helping me build my own net dream :) !