Hi Mark,
I agree with the intent, but pretty hard to execute in-camera. If one simply spot-meters faces then the exposure would have been about the same - which is close to what the floor is. In reality the flash is probably hitting the floor too - as well as causing issues with over-lighting the artist to camera right -- so to get it better in-camera one would either have to use a number of flashes and start flagging their output which is pretty hard without assistants and all the right gear.
I'd suggest just making the changes in post-processing. In this quick adjustment I've knocked the floor back 2 stops - the lights at the top back one - the person and area to camera right back one - and applied a vignette to try and achieve the isolation you mentioned ... (be sure to click for larger version)