Beware: This part is a cludge. It works for me, but YMMV.

It allows me to store links the normal way (C-c l) as mairix links and
insert them into my Org files normally (C-c C-l). I can also always
insert the latest SENT mail into my Org files by doing (C-c C-S-l).

Please consider all my stuff under GNU GPL v2 or later (v3 explicitly
allowed) so if someone wants to grab this and turn it into a "clean"
solution, that would be greatly appreciated.

Regards,
Georg


P.S. For completeness, and to make your life easier in trying this, this
is my .mairixrc file: