I tried adding it to the extension packages through system settings, and then commenting out the addPackage line in the main class.
No joy. It still doesn't find the Subscriber class. I'm positive I entered it correctly as well. Also cleared the core/cache/ folder to be sure.
Something very weird is going on. I'm on the latest version of Login and Revo by the way, don't think I mentioned that.
As far as I can tell there's nothing wrong with the package? I've compared it to multiple addons and the syntax matches up. :S
EDIT: Just did a 33rd test (judging by user ids) and removed the
var_dump(class_exists('Subscriber')); die; line from the postHook. Now it went through just fine. Apparently the class itself doesn't exists, but MODX still finds it when it's trying to create the object. So the extension package solution actually worked.
Atleast I can continue with my project now, but something is clearly going on with FormIt/Login hooks and addPackage. The overhead as Mark mentioned is a little disappointing, but this solution will have to do for now.
Thanks a bunch to everyone who have posted!
[ed. note: edgefive last edited this post 14 years, 4 months ago.]