Why the Brachialis?
Any muscle is possible - you would need to be able to see and/or feel where to administer. It is a smaller volume muscle, I doubt it would take very much to get the results you want.
While I'm certainly not the expert on this subject on this board I would imagine that you might have a hard time getting the results you want.
I would imagine that the only place you would successfully be able to administer and SEO in that muscle would be below the elbow. For the most part the muscle size from any SEO is going to be at or near the area of injection - the entire muscle doesn't just grow because you inject oil into one part of it. There are some good threads on here that while they don't address that part of the muscle explain how/why SEOs work. Might be worth reading.
It sounds to me like that part that you want to be larger/thicker is the part that is above the elbow and around the bicept. Unless you are very large and defined already I would think that would be a difficult area to target.