Can I use a camp fuel butane bottle with a brazing torch?
Butane has a much higher boiling point than propane (31 F vs. -44 F).  So if you're burning it at a fast rate, the pressure will be different.  If you're working in freezing temperatures, you may even have trouble feeding the torch.  

Also, I've read that butane doesn't transfer its heat as effectively (see this for example).  They have similar BTUs, but the rationale is that the butane flame is more diluted because it takes more combustion air.  I have no idea whether that's true.  That wouldn't make any difference for a camping stove, but brazing steel is more demanding.  

Nozzles designed for butane are different, and a different air/fuel mix is required.  So a torch designed for propane may not give the best butane flame.  All the torches I've seen designed for butane are small, but designed to get the optimum flame from the butane.  

The canister connection may also be different depending on size, but is you're asking the question, I'm guessing that issue doesn't apply..  

You want to use it for a demanding application.  The only way to tell if it will be effective is to try it.  It may be good enough, but it isn't likely to be as effective with a torch designed for propane as using propane.  If it does work, you may go through a lot more butane for the job than the amount of propane that would be required.