That's...really clever, but at the same time, I feel like there's a lot of assumptions baked into how Content-Types are used, and making your own content-type for each data model when it's all just application/json seems...wrong to me on an intuitive level, but I can't quite annunciate why.

I only half agree with the sentiment that /api/v1 violates REST patterns. I don't think there's any guarantee that /api/v1/bars/123 can't be the same object as /api/v2/bars/123.