Brion-
Hmm, would you consider something like {{ISBN}} to match the current variable replacement syntax?
Originally I wanted to use $isbn, but both this and {{ISBN}} have the same problem; if I use them in a link:
* [http://www.foo.com/bar?%7B%7BISBN%7D%7D bookshop]
The link no longer gets rendered on the [[Wikipedia:Book sources]] page (no proper url regex match). Although it would still work after the replacement, I considered that too confusing.
I've put it up on test.wikipedia.org (slight code style tweaks and potential apostrophe bug fix in CVS), please play with it.
Doing right now ..
Regards,
Erik