the dynamic-sample-loading branch has been merged into master. I will follow up with a few more pull-requests that add more bells & whistles to the feature, like the preloading function, but also some changes that reduce the memory consumption of the structural information of Soundfonts.
I would be great if you could try out the new feature by compiling and testing the master branch. Especially tests on platforms other than Linux would be great. I've done preliminary tests on Windows (which seem to work fine). But Androind, OS X, iOS are still untested.
Again, to test the dynamic sample loading, you only have to set the "synth.dynamic-sample-loading" setting to 1, either via the command line with -o "synth.dynamic-sample-loading=1" or via the fluid_settings_setint API function.