Finally I have some springtail shots taken with this setup. All but the largest here were taken handheld, with body-mounted Speedlite 600EX-RT; the latter was taken using a tripod, and the 600EX-RT triggered remotely by a body-mounted 90EX.
I've taken a lot of lichen macros this month with the EOS-M and MP-E, and it certainly works as a lightweight alternative to a DSLR. It's not precisely <i>comfortable</i> after long periods, but I think it works better. It's also good not to have to swap lenses on my main camera, which I use for birds and landscape shots when I'm out with all my gear. The touch-screen-to-take-photo option is very useful for minimising movement on the shot, which can ruin focus and framing at extreme magnifications.
The downsides? Much slower shooting speed - no way of doing two or three in a burst that I've figured out (but I haven't read the manual yet, oops). The images are visibly softer, given the lower diffraction limit compared to the 5DIII (f/6.8 vs f/10.1 according to Bryan Carnathan), and high ISO performance is of course worse, but I just need to remember to increase the flash power in that case. Cleaned up and shrunk a little, they look great. The combination is ideal for mobile macrophotography, I'd say.