I'd like to give readers the choice of where the
ISBN links go to.
...
It's actually easier to implement as a special page as Brion
points out: ISBNs will generate a link to
"/wiki/Special:Booksources?isbn=XXXXX", and that special page
will have links to Pricescan, AddAll, Amazon, or whatever else
we feel like adding, and even text explaining the different
benefits of each site.