Actually, there’s an obscure feature by which you can send “special” strings as control values, e.g. “c7” will set (actually map) the value by reading from control bus 7 and likewise with the “a” prefix for audio busses. I’ve used that here. But other than that, there’s indeed no support for any other fancy expressions as values in the server protocol.
This feature is documented in the server reference:
floating point and integer arguments are interpreted as control value. a symbol argument consisting of the letter ‘c’ or ‘a’ (for control or audio) followed by the bus’s index.