but the problem is it's a large forum
Well, our software is called "mini"... it's basically for a small-to-medium forum. I'm not guilty that it could handle large forums as well :)
I can develop mostly everything, but I must be sure about the thing I develop. As I've mentioned, the problem should be solved in its root. The Premoderation add-on, obviously, seems just not for your forum (and I'm sorry for that).
To develop something which would help you, I could suggest what I mentioned above. Having opened registrations for the massively spammed forum, you will never
get out of this problem, if you don't have resources to moderate such amount of users.
All the customers I know, solved this problem some other way. Some of them introduced paid membership. Some introduced invitations. Some forbidden public email domains for registrations. It all depends on what kind of spammers you get, what they do etc. I have simply no ideas about what's happening on your forum. But there are quite a lot of ways to solve it. Please consider them more carefully, and only then refer to my private support. Thanks.