Anyone know how to get a special duty assignment without going through branch?

I find the "power" given to Branch Managers to be one of the biggest issues with the Army's personnel management system. These programs---Drill Sergeant, for example, or the range of other special duty or broadening programs, are designed to benefit the whole Army, are DA-level processes, so why should the parochial, small-minded interests of one Branch give that Branch a veto over a higher level process. Let folks compete, and let the selection boards choose the best candidates.

Even if you don't start with branch it will always be approved by branch

Branch has to release you as far as I know, but if you volunteer it's after you apply and it drill sergeant is a powerful entity

contact the Drill Branch and let them know your are intrested for this duty. If you make the cut, i.e. Background check etc. then you the might be able to set you up with a school date.

We don't go through Branch in the Reserves. Getting Drill Sergeant school and recruiter and things like that in the Reserves is not difficult at all as long as you meet requirements and join the proper unit.

Submit a DA form 4187 at PAC with attached recommendations.