Since git looks at .gitignore and .git/info/excludes and bzr looks at .bzrignore, it might be a good idea to automatically symlink .git/info/excludes to .bzrignore.
In some .bzrignore files there is stuff like ./autom4te.cache but in .gitignore that should be autom4te.cache so maybe there needs to be some automated conversion from .bzrignore to .git/info/excludes instead.