As said - anyone out there got some useful experience on this?
Not exactly. My Fujin has only ever been used for racing and day rides, and I'm a lightweight. I find the caliper brakes OK (I did put Salmon pads in both). There's no getting away from the fact that the cable to the rear brake is long, though, particularly if you have the open cockpit.
Two points of anecdotal evidence:
1. Riders of fast machines (Burrows lowracers and the Wooden Fish streamliner, for example) in the BHPC often use only a front brake (and make sure it's a good one, like a Hope). Some consider an effective rear brake a liability.
2. The last time a rider on a lowracer applied a hydraulic rear brake in an "emergency stop" situation in a BHPC race, he ended up leaving quite a lot of skin on the tarmac.
I would look into the possibility of changing the fork for the aluminium disc-tab-equipped one before I would consider putting a disc brake on the back.