The only defined controls I see for reverb and chorus are for Reverb send and Chorus send (CC 91 and 93). That could be used sensibly for reverb and chorus send levels. That still leaves Reverb room size, damping, and width. With chorus it leaves the number of voices, speed, depth and type. Other CCs could of course be selected for these additional parameters, but that wouldn't be standard and might cause confusion if someone is trying to use the controls for something else.
Thinking about this more, I like the idea of having a Sysex interface to the FluidSynth command shell, which would open up a whole world of possibilities. Seems like the MIDI standard doesn't really have a "unregistered" manufacturer ID though for Sysex which makes things awkward and I don't think it is worth trying to buy our own ID for $200/yr.
NRPNs could be used potentially as well for reverb and chorus parameters and could provide a wider value range than the typical 7 bit MIDI values that CCs usually suffer from (unless treating them as 14 bit).