You could always do it in template.php.
Thanks for the suggestion! I hadn't thought of that. However, I am really trying to keep everything encapsulated in a module without requiring users of the module to change a theme or patch core. I did have one thought but quickly discounted it figuring it would make for REALLY ugly hack: 1. Give the taxonomy_breadcrumb module a weight such that it's hook_menu gets called last. 2. Access the global menu object (hack!) and overwrite each callback with my own (hack!!). I'd make the first callback argument the "real" callback function's name and use arguments 2 - N+1 to contain the original N arguments used by the original callback. 3. In my callback that would ALWAYS get called, I'd then look at the first argument to find the "real" callback to call, and then pass the rest on to the function indicated by the first. Following return to my callback function I'd then be able to do whatever I wanted to the page and be assured I was the last to have control of the breadcrumbs (and anything else on the page). Perhaps it would be better to try and get a new hook put into core? Being relatively new to drupal I'm not sure of the process or historical context of adding hooks to core. Do I simply write an issue and submit a patch--now or after 5.0 is released? Do I need to drum up support for it?