I am not the first to try SPD's on a unicycle, I have even tried them myself briefly, but not on a Muni. So I went for a ride with some friends to Hamsterly Forest today to find out how well they performed on my Schlumpf'd 26" Oracle.
My initial impression in the car park was that they worked great, I could change up and down with no problem and had no problem clipping in and unclipping.
My fellow riders were both supportive and unsupportive of my experiment. During our normal gladiator session on the way to the trail-head Joe Baxter just pointed at my feet and said "I think you are about to loose...." So I avoided him very quickly!
The first section of the trail is mildly technical with slimy clay mud and rocks going up a hill. I made it only 10m when I tried to correct in the first deep mud section and went down... still connected. Yes, they laughed!
It was not the only fall that I had, but as the ride went on and my confidence increased they were less frequent. The big advantage was that they were great on the climbs where they gave considerably more and smoother power. They also were great on slippy rocky sections where the wheel kept better traction due to the smoother pedalling strokes as I did not need to concentrate on losing my feet on the pedals. In sections of deep mud or very changeable conditions as on the top of the moors they were less good; here I would normally have dumped a foot down to transfer direction and recover my balance, this was not possible. I also found that on the sections where you needed to "have faith" that you could ride; like narrow muddy ruts or on big drop-offs, I would wimp. Although a lot of these will change with more practice I am sure.
So to conclude, good or bad... I would say it is ,mixed. I am certainly going to be using them again, but I will be looking for a pair of single sided SPD platform pedals to use so I can switch in or out depending on conditions. One piece of advice for anyone else trying this, make sure you put on wrist-guards and expect to fall more.