I think it's difficult to adapt mobile Artillery to the current spotting system in FH2.
If you notice when you place a mortar, it is always perfectly level, even it means parts of the mortar need to be sunk into the ground a bit because of uneven terrain. The very nature of truly mobile arty (Wespes, Priests, Sextons, etc) mean that they cannot always be level, which means the current arty system can't work with them (the dials and such would be inaccurate because the gun isn't perfectly level). Another problem would be constantly updating the dials every time the the vehicle moves the slightest bit (perhaps because of the recoil of the gun, or just the normal sliding down hills that BF2 vehicles suffer from).
I don't know what the Devs are doing to try to work around this (or even if they are trying right now), but there are a few suggestions to make it work that I've seen around the forums. One suggestion was that, by pressing a button, or entering a certain position on the vehicle, the entire vehicle "deploys" itself to be level, which might lead to clipping into the terrain and other undesirable things, but it would make the current arty dial system work (in theory). I think I saw someone suggest that the whole vehicle actually be immobile and replace the stationary Arty (LeFH and 25 pounders, etc) on some maps, but that's not really desirable, because it deprives the vehicle of it most important strength: it's ability to move. Another solution would be to make an entirely new spotting system that doesn't rely on the gun being level, but that would be time consuming, if not impossible.
I might be the first to suggest this, but is it possible to "trick" the game into thinking the gun is always level, no matter what? Like: even if the gun is not level, it still fires as if it were. I'm not explaining this very well, and I'm not even quite sure what I'm trying to say. Sorry for not being clear. But, I'm kinda tongue tied trying to convey what I'm imagining in my head.
I'd love to see truly mobile Arty in game at some point, but there are a few big challenges to overcome first.