Software for layering and selecting areas?

My application is a sales-territory-game sort of thing where users of the map will be moving around on foot within a local area.

To reduce bandwidth cost, I plan to use as many tricks as possible, including cutting back to 4 bits/pixel indexed color, not providing more zoom that needed, etc.

One thing I need to do, since the areas are self-assigned, is to allow the map users to claim an area by using their mouse to define a polygon in unclaimed space and save it off. The various claimed areas would then appear as 1-bit indexed overlays, the areas claimed by others in grey and the area(s) claimed by the current user in some other color of choice. I hope I’m not going to find out the hard way that what’s easy in Photoshop is difficult or impossible in OSM.

What I’m wondering is: is there software already written that allows a user to create a polygon with mouse clicks while disallowing infringement on space already claimed by someone else? Or will I have to write my own? I’ve googled on all the terms that I can think of without finding anything.

Is the following something similar?

Thanks! I think I might be able to steal some code from the MapCraft project.