I've just finished up my branch of Catalyst::Controller::DBIC::API. What a beast. It is functionally complete and backward compatible, but I will likely continue to tune it, write more tests, etc.
For my work project, I was bumping up against some pain in the ass things when trying to make DBIC::API play nice with Extjs's grid stuff. I needed some more configurability in what the data looked like going to and from the server. This included properly handling JSON booleans as well.
So I embarked on an ambitious task to add those features in, plus Moosify it. I also wanted to do more validation up front before ->search() was ever called. You wouldn't even know if your ->config() parameters actually fit your ResultSet until the first request came in and you watched it bomb.
The validation part took the longest as it involved the most yak shaving. The outcome from that though, is Data::DPath::Validator. Making use of Yuval's wonderful Data::Visitor, I wrote a parser that generates Data::DPaths, which are then used (ala Data::DPath->dpath) to validate data. So you feed it a template (replacing the allowed, variable pieces of the data structure with asterisks) and it will create all of the valid paths for the template, then feed it a data structure to see if it is valid. By default it operates in loose mode so only one path needs to succeed for the data structure to be valid, but you can easily flip it into strict mode where /all/ paths must pass for the data structure to be valid.
What this tool gained me in DBIC::API was a way to verify everything from search parameters, to prefetch parameters based on the provided config options (which were templates). I admit that my tool was more general purpose than the limited use in DBIC::API, so I had to subclass and flatten some aspects of the generate DPaths.
Beyond that kind of validation, I shifted goodly chunks of the request munging stuff into its own class with triggers for validating against the schema of the model. So most of the aspects of validation happen in one place, at one time, and very early in the request.
Anyhow, you can check out branch of DBIC::API here.
And you can take a look at Data::DPath::Validator on CPAN here.
Or even the github repo here.