We do not limit gear configuration on tech dives in any way, but we look at homebuilt/heavily modified rebreathers with a very close eye. We do not allow full sized doubles on any recreational dive trip, purely as a matter of space. I would have no problem with a diver bringing small diameter doubles (40's, 43's) and diving them sidemount, or diving an 80/100/whatever sidemount with a counter balance or not, and I have no issue with monkey diving. As Bob said, we look at sidemount divers very carefully, but only because it seems like the wrong tool for the job when diving 40-80 foot deep coral reefs.
I ask because I'm trying to get my head wrapped around sidemount for recreational divers. It seems we are going to see this configuration more and more, so I want the crew to be ready for it. It means I will have to go out and buy a bunch of 40's for rentals, and figure out a way to secure them on the boat. It's gonna make surface intervals longer, which means less diving for all because it takes longer to fill 2 tanks than one. Not for the air compressor, but for hooking up the whips and actually filling them. If it were a passing fad, I wouldn't bother with it. If it's something I have to cater to, I need to configure the boat a little differently.