How easy is it to insert your own generators into a schema in cases where one can't be automatically created? I'm thinking specifically of pred schemas or Java classes as schemas.

Configurability was a major design goal. Check out the test for examples of how to insert your own leaf generators or wrappers:

https://github.com/Prismatic/schema/blob/master/test/clj/sch...

Does that seem like it will meet your needs?

Author here, would love to hear feedback, and more than happy to answer questions