The Fluidsynth library is included in the Elk images, at the time of writing we used the latest 2.0.7 stable version which has full support for modulators.
Since the original plugin relies on the GUI to load a specific soundfont, we added the possibility to pass a path with an environment variable instead.
So for example, in the same shell used for run SUSHI you will first have to type:
$ export JUICY_SOUNDFONT_PATH=/udata/soundfont/FluidR3_GM.sf2
obviously replacing the file with the soundfont that you want to load at startup.