Thanks for the quick repsonse, Adam.
I found the js at the bottom of tag_image.ascx - I made my own copy of tag_image.ascx, hid your browse button, added my own, made sure to call FireUIChanged after returing from my browse dialog, and updated the tag.config file to use my version - everything worked great!
Updating the Style Builder Background Image browse button to do the same now...so far so good.
Michael