Hey Pete,
I'm actually playing a Fernandes Revolver Pro 7, which already has a sustainer. I'm just kind of planning on my next guitar which will need a custom 7 string sustainer simply because i don't really want to take the sustainer out of my fernandes. (my next guitar will probably be a custom built one because i want all kinds of abnormal features like 7 strings, a longscale neck, a U-bar tremolo and ofcourse the almighty sustainer)
Anyways. My band plays gothic doom... Doom meaning we have reeaaalllyy slow parts .. (60 bpm and less) and the sustainer keeps the chords from dying out. Plus i use it a lot for high squeeling feedback which sounds really cool before you do a pickslide. And i use the sustainer as a handy tool aswell, for when i'm grabbing my mic and singing with my hand on the mic or having a drink while playing riffs with only my left hand (people ask me how the hell i do that, you know the secret )
Our bassplayer has a 6 string bassguitar which doesn't have a sustainer but he uses the feedback from his speaker to do what i do with the sustainer but he'd actually like to have a sustainer on his bass aswell.. In the newer songs there are parts where we'll both just hold a note for a second or ten before continuing to the next part
Anyways, as soon as I have more news from my guitar tech friends i'll get in touch with you through email. I heard they were trying to analyse a Fernandes sustainer kit to see what parts they would need to build something similar and maybe even improved.