Hi,
I would also be very interested in a better sounding reverb.
I've written a different reverb as well, implementing a "sympathetic string reverb" using tuned comb filters. Currently I maintain it in a custom fork, but I wanted a maintainable solution. So I started working on the LADSPA plugin system: getting it working properly and in a stable manner, documenting it and tuning the performance. The largest part of the changes is already merged in the master branch on GitHub. So adding a different reverb would be as easy as writing a LADSPA plugin, disabling the internal reverb and configuring the new one as a plugin. But I guess that won't be much use for people using FluidSynth on non-Unix platforms...
So maybe what we need is a system where we can have multiple reverb implementations in core FluidSynth and a setting that can be used to select one of the implementations.
Cheers,
Marcus