I would agree.
The difference between the occasional double and pilot international duty limitations is that those are permitted by 121, and the "routine" of having open shifts which dispatchers are JA'ed for, after working a shift, to me also violates the INTENT of the regulation.
How often do open shifts occur? To me that is the question. My experience was also at a very small (5) shop, and in the year I was there, only occurred once (as recounted above).
If its only (on average) a once annual thing, then I would say that that is ok. However, if it is say a monthly thing, to me that seems like the intent of the reg is being violated, for that would sound like they have insufficient people to cover the flight schedule as currently staffed. Reorganize the workload, since you say that the 2nd shift may not have any actual releasing, just flight following.
Even at my 5 DX shop, we always had at least one overtime ho in the shop who was more than happy to pickup OT.