ahobson/ack-mode.el — comment on commit 846ed95e35ca5ced0406f6dbb2d41f6ebe6d1276
File:  line 0
By sudish:

Yes, make-variable-buffer-local will use the initial defvar settings as the defaults, but treat any subsequent set as local to the current buffer. That's perfect for this scenario. Do you want to try it and push me the pull request?

Nice use of dir-locals, btw. I haven't used the yet, but this seems like a great use for them.