Cheers Mark, great info.
Would rather avoid proprietary parts. But then at the same time I probably won't be needing to replace parts as much as with a regular geared MTB given single speed's robustness (so I hear) and it won't be mud churning through the Winter and I probably won't want to do my 8 mile round trip commute on it too often either. So still tempted by the Czar ION, it's lighter, disc brakes no extra, and I mostly wear shorts .
I did wonder if the SLX brakes might not be quite right for trials, so glad to have asked that.
It might come down to how much the cost increases with disc brakes are on the Flow. I haven't used non-disc brakes on a bike for years, can't imagine going back to not stopping in a hurry, but they must work well enough for trials right?