I have an aluminum single-speed frame that was marketed as a fixie. I'm wanting to convert it to a single-speed commuter. (I'm getting older and the fixie doesn't agree with my knees anymore.) It has enough clearance in the rear for 700x35 tires. It has a brake bridge but it doesn't have posts to mount v-brakes or cantilevers. Is there any kind of adapter that could make that work?
Adapters like this do exist, e.g. http://www.danscomp.com/489051.php?cat=PARTS or http://www.bentechbikes.com/vbrakes.htm (Edit: Links are now dead, but they were basically the same as the one in the photo below. Sourcing one might be even harder now though.)
In fact I used one on my old folding bike - but you can see it looks a bit of a mess, and when I tried it on the front fork, it really wasn't up to the job (I ended up using a chunky long reach caliper brake taken off a BMX bike). (I'd changed wheel size and was using much fatter tyres, so the original brakes wouldn't fit.)
There are also adaptors that bolt around a chainstay: http://www.ratrodbikes.com/forum/viewtopic.php?f=1&t=33168&st=0&sk=t&sd=a&start=30
I'm not saying I'd recommend you use them, but the option is there (if sometimes hard to find - the one I used was the last the local store had in stock of a discontinued line).
I do not think anything bolted on could be good enough. Since it is aluminum frame, brazing or welding the bosses would also be complicated.
But instead of replacing the whole frame, you might just change front fork with one that has bosses. That way on front where it makes more difference you would have V-brake, and on the rear you could have long reach caliper brake.
They are not cheap ($130 front & rear), but Campagnolo makes a lateral pull TT brake that mounts on the center post:
But, wouldn't it be easier to mount side-pull caliper brakes? You can buy a pair of good-quality dual pivot side-pull calipers for $40-$60, and they will just mount on the single-bolt brake bridge you already have.