[quote=@Unoedipal]
<Snipped quote by IncredibleBee>

...was there something in my post that didn't explain why that's impossible? 

I'm legitimately confused by this response.
[/quote]

Yeah but the grail's a magical genie wish granting device as it is, so I'd say it could pull out some spare prana.
May also be that an especially wily Heroic Spirit managed to force itself into the Grail War, perhaps forcibly sucking mana out of Fuyuki's ley lines.