I know you specifically asked about using a page callback function, but the more general question (emphasized in the question title) is simple to redirect a taxonomy term to a custom path/page. For that, the Page Manager module (part of CTools) is a great tool. It may help many people who want to accomplish the generic case, but don't know how or simply don't want to use a page callback function. There's also a great set of videos about Page Manager on Drupalize.me. At the time of this writing, those videos are free, not sure if that will change at some point.

I know you specifically asked about using a page callback function, but the more general question (emphasized in the question title) is simply to redirect a taxonomy term to a custom path/page. For that, the Page Manager module (part of CTools) is a great tool. It may help many people who want to accomplish the generic case, but don't know how or simply don't want to use a page callback function. There's also a great set of videos about Page Manager on Drupalize.me. At the time of this writing, those videos are free, not sure if that will change at some point.