Yeah. I actually tried that too, but it's just as problematic. I was assigning that Action (just as you described) to the "Music" button on my remote, and then assigning a similar action to my "Video" and "Photos" buttons. The problem is if you hit Audio (which selects Audio and opens the tree) and then Video, and then Audio again (to go back), it jumps you down to the next "tier" of the Audio tree (in my case an [Album (grouped)] view scheme that I only rarely use).
The SendMessage method actually worked more reliably than that. It would only get confused if I toggled back and forth quite a few times (10-15 in quick succession). Once it did "break" though, it would actually start expanding the wrong tree item (seemingly at random) so it was quite unacceptable once it started happening. I couldn't find any way to "set it right" other than to close and re-start Girder.
I decided it's easier just to manually hit -> to expand the tree myself when I want to open it, and left my Action with "Switch to Audio" alone (along with the conditional launch MC or bring it to foreground). It is working well though.
I also thought I'd mention that I had a lot of trouble getting SendMessage to work reliably with Theater View (I was assigning those same buttons to do the similar function in TheaterView if it was open). It works most of the time, but sometimes would get "stuck". I suspect it happens if TheaterView somehow is no longer "on top" or something (or has to do with the Target I was selecting, but I tried a bunch of combinations). Either way, I've decided it's just easier to use MC11.exe (and Keyboard spoofing) to control Theater View at this point.
I'd be all done with the GML right now but I decided to break my HTPC (not really on purpose) yesterday instead. I plan to work on it more tonight, and I should have something post-worthy by this evening.