Is there a Regulation that prevents my unit from sending me on a rotation?

No. But there are Compassionate Commanders. And then tend to be even MORE compassionate if you do your homework.

Lay out your plan. Show what the requirements are, which requirements you have met, what is needed to meet those you have not yet met, and what your plan is to fulfill those needs. Also lay out WHY that MOS will benefit you, as well as why you, as an individual, will be a ctrdit to that specific MOS, and to the Army as a whole while serving in that MOS.

There are no guarantees, but if you do your homework and can show the Commander that leaving you off of the rotation WILL produce your desired results and it is in the best interest of both you AND the Army, you have a chance of being left back.

As a caveat, work this up through your CoC. Present you whole pitch to your SL, amd ask for his/her input as to what you need to work n (strengthen am argument, provide more evidence, work on organization, etc.) Once SL says, it is good, present to PL/PSG for endorsement. If they agree, ask for THEIR input to get even better before pitcjing to Commander. Having PL/PSG buy-in is critical when you pitch the Commander. Applying their feedback is equally critical (and can garner a stronger endorsement from them once you leave the room).

Right now you're doing a voluntary Reclass. Since you're asking about BMST that means you're requesting 12P. There's no guarantee that you'll pass the BMST so you don't meet the requirements of MOS yet. But the Army still hasn't issued a DA Directed Reclass for 12T's yet either. Right now, the 12T's at the greatest threat are the ones in their reenlistment window. If your rotation is Korea there are still testing centers you could go to. If you get promoted you won't be under precision retention anymore either.
The other option is to pick another MOS, there are plenty of others to choose from.

There is only one that I know of, your just returning from Korea then your exempt from deployment for a year. this happened to my when I returned from Korea in 2009. the unit tried everything to get me on the deployment but the Department of the Army said no. Other wise enjoy your deployment.