Playback of string tremolo
Currently, you can use the tremolo soundfont for strings, but you have to add system text. The actual tremolo symbol just plays the attack repeatedly (which sounds bad both in regular and tremolo soundfont), making it impossible to have a score that both plays back and displays tremolo. The playback on the tremolo symbol cannot be disabled.
There are workarounds - you can use invisible notes set to the tremolo sound and then mark the notes with the tremolo symbol as "Don't Play" with the inspector.
Or you can remove the Zita reverb which improves the sound of tremolo considerably.
As an alternative to the tremolo symbol, I'm using the staff text "tremolo" with property set to use the tremolo soundfont, also a line to indicate the extent plus the word "end" (with "normal" soundfont property) for the end of the tremolo. This is how it looks:
The word "tremolo" can be repeated at the beginning of each new system (e.g., for a new page), but without re-setting the tremolo-font property (since it's already set).
With this notation I don't see a need for the tremolo symbol, which is ambiguous in any case since it can also indicate division of a note into a specific number of bow strokes (also valid, but a completely different form of ornamentation).