Are there any additional sources of martial arts rules other than the GURPS Martial Arts source book?
Not at the moment. At one point, GURPS Martial Arts Adventures was in print, which contained contains several pages of additional martial arts rules. However, these rules were assimilated into the expanded second edition of GURPS Martial Arts, as were several Roleplayer and Pyramid articles dealing with martial arts rules. Both Bunnies and Burrows and Lensman contain setting-specific maneuvers and styles, however.