Don't know if it's any interest but I built up a non-disc frame Cannondale with a disc Headshok a couple of years ago when I managed to get a full set of Maguras for £8 from Perth Bike Station. Used a Julie disc brake on front and the Magura rim brake on rear. Didn't want to have 2 different levers on bars so just used the Julie lever from rear caliper with rear Magura. The lever and rim brake combo worked perfectly. Never had hydraulic rim brakes before, couldn't believe how powerful they were, only minus is I forgot how crap rim brakes are when wet even with hydraulic power.
Just remembered though that Julie lever/master cylinder are plastic/composite, black plastic oval reservoir cover, with a 2-finger aluminium lever. Trying to remember but I think that the earlier Louise/Clara lever with black plastic oblong reservoir cover had an aluminium body (but plastic lever?) which would be easier, I assume, to paint versus the composite. Will have a wee look to see if levers can be changed around between each system's master cylinder.