I think just adding something like the "Filter Selection" option of NEdit would go a long way to solving issues like this. You can select some text, pipe it into an external command or shell script, and replace the selection with the output of the command.
For instance, you could use something like this perl script to emulate the "Justify Paragraph" option: http://directory.fsf.org/all/rewrap.html
But "Filter Selection" would also allow for all sorts of other user-specific filters, without adding a lot of bloat to Geany, almost like a sort of "plugin" or "macro"
PS: Sorry if you got this twice, John - I still haven't gotten accustomed the geany list not munging the mail headers. All other lists I subscribe to use a Reply-To:<list> header.