Jesse et al- SL is fantastic. I've pulled my hair out trying to build essentially the same thing in Max/MSP, but having SL to use just takes all the work out of it and lets me focus on starting to make music.
It makes me so happy that I'll even make a donation if there are a few things you can address. I'll even do what I can to help solve the problems, considering that you don't have Mainstage.
My goal is to have something simple so that I'm not switching between windows i.e. entire interface will reside in Mainstage. The problem I'm having is that certain mappings in Mainstage are either inconsistent or don't seem to work at all. For instance, I'd like to replicate the 1/2, 1, and 2X buttons from the SL interface. So I made 3 buttons and mapped them to rate. No matter what type I set the button as (absolute, single value, etc.) it would require a minimum and maximum value. Which is fine (I would just make the min and max for each the same number), however there's inconsistency in the response. Sometimes it takes multiple triggers for one of those actions (going to 1/2, single, or double speed) to occur. And often the buttons state (pressed or not) does not reflect the actual state in SL.
Also, there does not seem to be a way for me to get individual loop level monitoring. I assumed it was "out_peak_meter" but that produces no response. Ditto with "waiting". I'd love to see which loops are "waiting" for sync, but this again does not produce a response in Mainstage.
Anyway, if these can be fixed (or my ignorance can be righted), then great. If not, then maybe I can forgo trying to run SL as an AU and just use Jack and Midi Patchbay to get my sounds and triggers to/from SL.