Thanks for the info, Ben. Apart from needing a new rim, the V-brake is pretty much zero cost as I already have all the parts.
It’s a tn719 which definitely won’t work with a rim brake - has a very slanted sidewall. I think the hub’s been rebuilt a few times to try out different wheel sizes, so it’s an old hub, but a fairly recent wheel build, and presumably the original owner didn’t envisage needing brakes. No worries about rebuilding - I built the wheel on my 19er myself and rebuilt the wheel on my unguni 29er when the spokes started snapping. In fact one of the main things prompting this thought is that I’m going to rebuild the current wheel as I found some of the spokes had gone completely slack, and then decided I might as well sort out the spoke pattern whilst I was at it (the valve hole is in the wrong place in the spoke pattern, and on one side the spokes are in the hub the wrong way - the spoke holes are only chamfered on one side on a Schlumpf). Will rebuild on the current rim for now as I want to ride, but looking at options to rebuild so I won’t put huge amounts of effort into making it perfect.
I’m seriously considering an Exal ML21 http://www.spacycles.co.uk/products.php?show=1569 as it’s that little bit wider than anything else I can find - I’m sure my brake mounts will cope with that, just not with much wider. Though it is rather heavier than the 19mm wide (internal) alternatives, if still a lot lighter than wide uni rims. I guess 19mm wide is fine for road use, and I’m not really likely to take this off-road that much - it would just be nice to have a bit more support for a widish off-road tyre to try occasionally, and I’ve seen good reports on that rim for strength, so the only real downside is the weight.
Oh and interesting thought on the Sinz disc cranks - hadn’t realised you could get those in ST, and the 135mm crank length would be interesting if that’s available in ST as I currently have 140 but nervous about going down to 125 (though on the downside I’m not sure I ever would have the option to go to 125, and changing crank lengths would be an expensive business in general).
Though we still come back to the issue of caliper installation - it’s an alu frame, so braze on steel mounts are no good, and I’m pretty sure a UCM wouldn’t work as the bearing isn’t centred in the mount.
135mm SINZ Square taper is available, and understood on the aluminum frame. The bearing cradle on UCM is centered, but your saying your frame would require an off-center mount?
It was using a system like the Mountainuni with the rotor attached to the crank, and then i used a cable actuated disc caliper. Its pretty straightforward. Actually its easier now with the advent of the lower cap caliper mounts (UCM/d-Brake). On mine i had to make a caliper mount that bolted to my frame.
Also, its not usually a good idea to post email addresses in a thread, so if you want to contact outside of a thread a private message is welcome.