Unlike other plug-ins (from simple like GenericSurface, throw A-PRO, up to VS-700), MackieControl is not working with ACT Dynamic Mapping (sometimes references as "ACT", as so confusing). It uses "direct" way to control FXes, and in general FXes (in FX bin) only, so no ProChannel and no Synth Rack. That is the software limitation, so it can be potentially changed.
C4 bound to MackieControl just extend the number of available directly accessed parameters. So it can control the same parameters you already able to control with other (MCU/XT) units, just with less page switching.
Note that all MC units should be able to control ALL FX Bin plug-ins, in "Plugin" mode, using corresponding button combination to switch between plug-ins and banks of parameters, see Help file for complete information.
What is mentioned as "the first plug-in only" limitation is true for ProChannel modules with stock plug-in, when MCU is in EQ mode (the second module in Dyn mode). There is (my) mod for stock plug-in which fix the issue and controls EQ and Compressor (default Sonar modules only) in corresponding modes, independent from there real position inside ProChannel.