As it stands right now when editing custom properties of a node you need to remember the exact way something is spelled, capitalized, etc which can sometimes be a pain, and if you don't get it exactly right it'll just create another one for you which is kind of annoying. I think it would be great to have a pulldown that lists all the current selections, along with one that says 'New' that lets you create a new one.
Overall I think it would be big time saver since you'd no longer need to remember everything, just pick it from a list..
I would like to see this also.
Seriously, it's a good suggestion. It's on the list.