Sensaphonics and ACS IEM drivers can have electronic ambient options via beltpacks.
I have to say though, speaking from professional experience mixing monitors for large concert tours and TV shows, these kinds of things tend not to be used.
I think this is mainly because a good IEM mix shouldn't need additional stage ambience to capture the performance element. You have mics in front of all the instruments right? If the guitar sounds dry because its close miced, add 'verb. If the drums sound too dry, add some overheads. etc...
Ambience, IMO, should really be there to capture the bits that aren't miced. Like the crowd. Most artists switch to IEMs to gain more control over what the hear and reduce stage volume. Adding back lots of local ambience mics would undo that pretty quick...
Chris, I readily acknowledge your experience and truly appreciate your guidance.
FWIW, my concept of a lav mic on the musician was to reintroduce some of the normal stimulus they would get if their ears weren't plugged, thereby hopefully removing some occsaional early-adoption objections to 'it just doesn't feel right' (I am working with a guy this weekend whose entire band is on IEMS,except him). The musicians would still have the benefits of isolation, stage volume control, and detailed mix control of IEMS, but they would also be able to dial in some of the 'pre-IEM' sound.
I didn't mean to propose this as the only ambience mic, just one possible ambience feed combined with near and far audience feeds.
Perhaps the lav-on-musician approach would work like training wheels for artists who don't immediately take to IEMS, and over the course of several gigs they would slowly dial down the local lav mic.
Just thinking out loud,... I frequently have ideas that don't pan out.