Iam running RF51 and would like to be able to export (archive?)
video recordings to either another area on the myth filesystem or to
a network share, and to remove the commercials flagged in myth.
Can this be done so that it is an option in the menus? mp4 would be the
preferred encoding but just outputting to mpeg2 without commercials
would be fine too.

The keywords you want are "transcoding" and "commercial cutting".

For information on MPEG-2 -> MPEG-2 lossless transcoding see this thread: http://mysettopbox.tv/phpBB2/viewtopic.php?t=15056 which contains links to the directions for setting it up.

For other transcoding setup see the MythTV wiki: http://www.mythtv.org/wiki/index.php/Removing_Commercials#Editing_Recordings