it basically would be possible to make every window and ledge like a vehicle you the can shoot out.
but the problems are:
- LAG/Netcode: BF2 allows only a certain amount of objects that are "tracked" by the server and their position is sycronized with all clients. this includes every vehicle (also static guns etc) and every moving part on them (including wheels,turrets,hatches, guns , MGs, etc), every pickupkit, every destructable thing etc etc.
adding too many of those will crash clients/servers and decrease general performance.
- you would need a special MG thingy to shoot them from those "window seats" because you cant zoom/iron sight with the MGs in those positions. thus affecting normal combat.
however the bigger problem is the lag stuff.