since these things are only used for indirect fire, they have to remain stationary for a turn or so to plot fire. One can assume the mortar is set up beside the vehicle at that point. (And save an element, actually 2, as separate mortars).
If you want to be pedantic about it - use unit #289 (bren carier APC class) which is currently unused, and make a section of 2 with 2 single 3" mortar as a separate formation, and set your availability dates to what you think it should be. Agree with your opponent to only use that formation and your OOB as appropriate.
If your problem is an opponent using sp-mortars as
direct fire artillery (which is ahistoric, but the game engine will allow), then simply do not play that opponent, if he continues to insist on doing so. (Or use any cheap tank to remove the problem, as they are thin tin and hence rather vulnerable.)
I never use Sp-arty or mortars in direct fire mode, my mortars (SP or otherwise) are always tucked away in dead ground. (But then me served in the bn mortar platoon

Exposed arty is soon to be dead arty, I have found.
So - several work-arounds available, rahther than a mass exercise in examining all scenarios and OOBS (UK, CDN, India, Poland etc all have the possibility of the item), just as the patch is being assembled this weekend.
Cheers
Andy