Shame to see you go so soon [@Endrance]. You could always try to ask Plank or Crimmy if you could submit your Master as an NPC of sorts who wanted to join the Grail War but showed up only after all Servants were already summoned. He could then try to wait around (or even try to orchestrate the death of a Master so that he can assume ownership of their Servant) and possibly become a Master in the future ^o^
That is, only if you want to of course.