I've just implemented the most requested feature in bzr-builder (Hi Ted), command support.
Sometimes you need to run a particular command to prepare a branch of your project for packaging (e.g. autoreconf). I think this should generally go in your build target, but not everyone agrees, and sometimes there is just no other way.
Therefore I added a new instruction to bzr-builder recipes, "run". If you put
run some command here
in your recipe then it will run "some command here" at that point when assembling.
Note that running commands that require arbitrary network access is still to be discouraged, as you don't know in what environment someone may assemble the recipe. I'd also advise against using commands unless you really need them, but that's obviously your call.