oddly enough this is an issue for me at present; my (early) APB was originally configured for an IGH, and has canti bosses. Because the 'seatstays' are so flat, moving the wheel to change the chain tension almost invariably involves moving the brake blocks in the rear brake too. This means there is very little overlap between the settings which allow 'good chain tension' and those which give 'acceptable braking'.
In fact the whole thing is set up in a weird way; at the front the canti bosses are at an unconventional height ( lower than normal), such that if I use a full-size V-brake it has to be with a standard road pull lever, because the brake MA is so low (the brake blocks are abnormally high in the slots) that the brake is nigh-on useless with a V-lever. At the rear this is also the case when the wheel is in vaguely sensible positions too. I can only suppose that there cannot be a canti which has a high enough MA to give good braking on this frame; leastways I have never found one.... yet there must have been cantis fitted to it originally, there were cable hangers front and rear. I even checked to make sure that the frame wasn't designed for smaller rims than those fitted, but it wasn't... all very odd.
So in light of the rear setup I am considering fitting a gear hanger and using a derailleur setup, a tensioner with an IGH, or some form of hybrid gearing, so that I can have a vaguely sensible rear brake more of the time. Some options will need FD mountings and extra cable stops etc. and all will ideally need a gear hanger welding on too.
cheers