You can also programmatically control almost everything via midi CC
messages and a carefully edited soundfont. That would work in your
case, although it's a bit of a faff to set up.
Right, so I tried adjusting the "Reverb" knob in Rosegarden (connected to FluidSynth) and didn't hear much difference. I'm not entirely sure what all the reverb settings do (there are four), but I found that increasing the "room size" from the default of 0.2 to 0.8 made it sound very nice. I'm not sure how to do that (change the room size) via MIDI commands. Perhaps that's why I need a carefully edited soundfont too.
In any case -- I succeeded in my hackery so I don't need this problem solved urgently. If there is really no way to change the settings within the -F fast renderer, then I will try to implement the proposed solution.