Thanks for your kind words about my site, Rob
I can't quite remember how I managed to get Fronimo fonts to work in Sibelius as it was several years ago when I did it. With some of them I think it was just a matter of making sure that the font files included a version in the right format (.ttf) though I may have used a font utility for converting to .ttf (the free FontForge should do it).
I've got Fronimo 3 which I run via VMWare Fusion, but I don't use it much. If I want to make an arrangement from a Fronimo file I use Save As
to create a Midi file and then open it in Sibelius. The result needs dropping an octave and for some reason the instrument is given as Pedal Steel Guitar - and of course it needs quite a bit of editing to make a sensible arrangement.
Have you looked at writing lute tablature in MuseScore? Their lute fonts look pretty good, though I haven't explored how to write the bass courses.