ah - yes, you wouldn’t have any trouble with the hydraulic Magura rim brakes, as they’re not actually pivoting, so the mechanical advantage hasn’t changed at all - unlike, say, running V-brakes with a big pad adjustment range (like Paul motolites) and just shoving the pads waaay up on the arms.
The question is do the Magura’s mounts have enough adjustment range to put the pads in the right place. I guess we’re only talking ~12.5mm of adjustment necessary…
I believe that a 27.5 hydro rim brake setup in the rear has already been disqualified because they will not line up. But that’s why I say go 26 rear 27.5 front!
I have a nice Brake Therapy rear disc conversion kit that could be for sale. They were basically made for this situation -people with nice old titanium “forever” frames built just before disc brakes came into fashion.
The kit I have is complete including the specific spacers and parts needed to use it with a standard Shimano cup-n-cone rear hub(these are very often missing). Includes instructions. I believe it could be used with other hubs but would need some research.
Not my picture, but this is basically what it looks like.
I got a cheapie Alloy Masso from AliExpress and it was 7 grams lighter than the carbon one you are looking at. The Carbon one WAS for a 29 inch wheel tho. The Masso was nominally for 26 but also fit 27.5 and even a 29 inch slick. I just covered it in wide black insulation tape on day one and it’s still covered in it.
I spotted a nice old 26er wheel for sale with a DT Hügi hub (laced to some 36h Sun rim) and took it home for $150. While futzing around with it trying to get the freehub off to take a peek at the 18-click star ratchet, I realized that this is some real old '95 style Hugi hub that needs you to take off a NDS axle adaptor (which didn’t want to pry off) before you can unthread the DS axle adaptor. While sending pics to @Crustradamus, he pointed out that the threading on the NDS of the hub itself was probably for some tandem brake and then, sure enough, it turned out this thing is a 140mm OLD hub. That explains why the matching front wheel to it the guy was also selling is 40H. I can’t find if DT makes 135mm axles for hugis this old, so I don’t even know if I should:
I assume you have seen these install/dismantle instructions?
Looks like from the parts diagram, the left axle end cap/adapter is the only thing that is OLD-specific. Of course there could be some tandem-specific fuckery going on, but I’d pull that cap off and see what it would take to drill/file/machine it from the current size down to 135.