Hi,
Injecting new objects into your game's memory (assuming you are the client) will not work, since your "bogus" data will be overruled by the server. It's a very basic anti-cheating thing.
Injecting your data into server's memory though *might* work. I once did something similar, and I ended up with a C++ program that would scan the server's memory searching for PCOs with known locations. Then it would track them, calculate their speeds, etc. And it would work only half of the time. I guess it would take someone with better cracking skills than mine

The heart of the problem is that, in Battlefield 1942, the position is not a simple thing. For every PCO, the engine seems to remember lots of past positions, the current position, and even some predicted future positions. It is very hard to identify which one of these is your actual position, and even if you get that right, the server seems to scan the position array for anomalies. If you inject a new position that is sufficiently far from the past position, the server will decide that such a move is impossible and immediately pull your PCO back.
All in all, I'd suggest finding a solution that does not involve memory hacking. How about those buildable walls from the Battlefield: Empires mod? They seem to do what you want, though it's not possible to "save" such a map.