Okay, good point, that was not know to me...

You would still have to rewrite that part of the server code and work around the native way to make radar usable with areaportals generating additional calculations and network traffic.
Only to make areaportals workable that are not really needed anyway. There are huge maps with high polycount like arachnid2 that work without areaportals (there simply are no doors) and draw reasonalbe parts of the map with radar working well in any situation.
You see: Well placed hint brushed do the same job.
Danny