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

I think the ritual for summoning a standard servant is already something that was understood by humans, so with enough time and experimentation you can replicate the process, even if summoning the eighth servant does require an atrocity (not that Nasuverse is averse to those).
Even if the servant's own dead body doesn't power the grail, this does mean that the cult in question now has a way to reach the Root without sacrificing their own servant, which means you keep your super strong magical friend a bit longer AND you don't need to sacrifice him to get the super secret bonus, which would keep said Servant from turning should it ever find out.
[/quote]

It's not, that's why when what the Fuyuki Grail actually did became public in Apo universe, everyone went "holy shit we thought they were joking". Also, the fact that the Fuyuki Grail's purpose is to reach the Root isn't public knowledge; even in the Apo universe where Grail Wars became a biweekly occurrence, that fact was only known to a handful of people in the World. 

I'll allow this cult to have managed to find a way to, via Greater Ritual, do something similar to the Grail's summoning on a much smaller scale and summon/bind a Servant independent of the Grail, then choose to send the resulting Servant/Master to Fuyuki so that they can reap the fruits of the real ritual. However, as a matter of course, the Container for this makeshift Servant will be noticeably worse than the container made by the actual Grail, as given by Apo.