This actually could be generalized to have a bigger problem:
We need standard API to do the following:
Ask some params from synth.
Set some params of synth.
Use a synth on sample-by-sample or chunk-by-chunk basis.

Many parameters of most synths can be controlled through MIDI, if that's what you're looking for. As for seamless interconnection, that's never really something that exists. ChucK even uses a sample buffer (of 256 samples by default), so any "mouse movements" that you're making are already getting delayed by that amount.

So, it looks like what you actually want is something like Pure Data (open-source) or Max/MSP (commercial product). PD has a standard object called [vst~] that you could load ZynAddSubFX into and get all you need out of it without ever touching a MIDI protocol. As it seems that chuck doesn't do what you want it to do, and probably won't in the foreseeable future, I'd suggest just downloading PD rather than begin to learn a programming language that doesn't natively support external synths.

Most people, I think, use chuck because they want to build their own synths from the ground up. If you want some tips on particular sounds, maybe check out the electro-music forums for sample code.

-Andrew