Thanks for all the detailed answers stvn. I already had one go at predefining types with M::U::TC, but with no success. I kept getting an error suggesting that I needed to call class_type, which I did, but the error didn't change whether or not I called class_type and defined the type inside or outside the class block.

As for MooseX::Params::Validate, it looks good, but getting MooseX::Method::Signatures to work seems more worthwhile, just because of the similarity to perl6 signatures.

I'll have another go with M::M::S next week. Right now I'm under pressure to produce a working demo, and I've pulled MooseX::Declare out of the code. If I can't get it to work, I'll be sure to submit a bug (if I can't manage a patch).