As the header says.
In general I like YAML more than JSON these days. I implemented a RESTful WS PoC back in teh day using JSON. I was wondering if I can instead use YAML or not.
E.g. are there enough tools/libraries/support for doing that? Or would I end up doing quite a bit of mundane/tedious coding which I would've avoided if I were using JSON instead?
Also as I understood from WWW: REST doesn't restrict one from using YAML as the payload, is that correct?
Yes, if it's a goal that the data be especially readable to humans. There's not a lot to gain here for webservices however, which represent app to app communication. Computers don't care, and JSON can be pretty-printed to improve legibility somewhat. REST itself isn't focused on protocols/formats so much as patterns.
I could imagine some use as a debug parameter, perhaps
…?fmt=yaml to assist during development.