Resurrecting this thread, I noticed the following news on Digital Spy:
'Next Gen' actors to appear in 'Enterprise' finale
http://www.digitalspy.co.uk/article/ds19247.html
Star Trek: The Next Generation actors Jonathan Frakes and Marina Sirtis will make guest appearances in the Enterprise finale, it has been confirmed.
Speaking at a UK conference at the weekend, Frakes said that the pair - last seen together in Star Trek Nemesis - would be starring in a secretive plot for the show's last ever episode.
Filming is due to begin shortly before the episode airs May 13 on UPN in the States.