just writing down this idea I just got, no idea if it would work or not.
If mobile artillery is not possible with the current artillery system since the gun need to be perfectly level, then how about this:
Rather than having the artillery's shell actually travel the distance to it's target, would it be possible to simulate the firing of the artillery, and then 1 artillery shell from "above" (read commander artillery) could fall down on the target.
if you get my meaning
1. move your artillery piece
2. Line up azimuth and elevation
3. Fire (it does not actually fire, you only hear the gunshot)
Edit:
I'm out of creativity right now, but how about using a different artillery system just for the self-propelled artillery? and still use the same for fixed artillery.
4. 1 shell of commander artillery drops down.
But perhaps I misunderstood the problem from the beginning.
otherwise, how about what Thamu said?
You drive it to the location needed,and you press one key,and you "drop",the Priest on the ground,move to 2nd postion and you fire,move to 1st position,press another key,and you can drive it again...I dont know if this is possible with the BF2 Engine...
You could perhaps only be able to fire on somewhat flat ground (not too steep), it would sink into the ground but not that much.