Look up Opera on Sydney Harbour (my current gig). The orchestra is in a studio underneath the stage (but not underwater!) with a big-screen behind the audience so the cast can see the conductor. I've done it in musical theatre before, but never have we done it in opera - however it's working surprisingly well. The players have had to get used to playing with headphones, but they have their own mixing desks for foldback (the Aviom system) so they can adjust what they're hearing - and of course they can hear their own acoustic sound, just not the singers. The conductor has FOH cam but no-one else can see the stage - however that's pretty normal for them. Of course, being an outdoor gig, the singers are all amplified anyway, so the mic'ing of the orchestra can be balanced precisely with the singers, and fortunately we have a brilliant and wonderful sound engineer at the desk - which reassured the orchestra quite considerably. There have been comments from a few punters asking why we can't see the orchestra, but we bring them on stage for a curtain call which makes a difference.
We're also doing something similar for an upcoming production of
Die Tote Stadt where the orchestra won't physically fit in the pit (they would if it wasn't for the celeste, harmonium, piano, second harp and bass bells) plus the set designer wanted to use the pit space. So they're in one of the smaller theatres at the Opera House which is, as it happens, a really nice space for them to play in. It will be a little trickier from a sound point of view because the singers WON'T be amplified (although they'll be wearing radio mics for foldback purposes) but now that the Harbour gig has opened successfully and the musicians know what they're up against, it will be fine. Fortunately we have the same sound engineer which will also make things easier.
We had some fun bumping the Harbour gig in - how often do you see this: