Welll the reverse of your logic is that we cannot also expect that just because there are not tons of photos of FG42s in the East, that doesn't mean they weren't used in numbers either.
If only around 7000 FG42s were made, 7000 is still 7000. Overall prodution does not matter so much as *concentration* in a given battle. If the opposite was true, then why are we getting the Tiger I in ROOST when only 1300 were made? Well, we get it because it was present at most all of the critical battles.
It follows then that it doesn't matter if the FG42 was relatively rare, the FJ in themselves were relatively small in number, so it makes sense. But, if you have several maps depicting major operations where FJ were present, then it is entirely appropriate to give them the FG42. I understand that ther eare more important things in ROOST, but down the line it would be a great addition, especially for map makers that want to construct FJ maps.