Oh, yes. Piss off a lot of people, that would, though different groups depending on where it happens.
Also, Asgard might have something to say about it, if they're actually temporarily resurrecting dead heroes, rather than, say, making temporary copies of them (which is closer to what the HGW does in canon). Might get their attention anyway, because a lot of the canon magi in the Holy Grail Wars are utter dicks.
... and now I find myself imagining someone in the IPO or WDF getting involved due to being on vacation, and suddenly the Command Seals appear, or alternatively, due to a dead friend or relative who didn't Return calling them up to say hello. Tali'Zorah's grandfather is a genuine Hero, IIRC.
"They planned their campaigns just as you might make a splendid piece of harness. It looks very well; and answers very well; until it gets broken; and then you are done for. Now I made my campaigns of ropes. If anything went wrong, I tied a knot; and went on."
-- Arthur Wellesley, First Duke of Wellington