While poking around in the Xchat command set, I found the settab command which allows you to temporarily (until the next join) rename a channel. A bit of further research showed a dbus interface to Xchat. From there, it was simple.
The configurable bits are the two "string" entries in the line starting with "context" and the "settab docs" entry in the last line beginning with "dbus-send". Note: if you make an error in the "string" entries, the channel name for the previous call will be changed. (You'll know what I mean when it happens.)
Chain a bunch of these in a single script and you can mass rename channel tabs. Add a desktop launcher for the script and you can mass rename Xchat tabs with a single mouse click. Enjoy!