Burke,
It looks like there only about 3 posts on the whole web about using JSON with the REST interface.
Nor can i find any documentation on it. I can't understand why the support seems to be so poor as the natural data exchange format over a REST interface is JSON. Granted in the POST body it doesn't really matter, but still...
Since you are getting the exact same error, it looks like you may be correct about the update not fixing the issue.
There doesn't seem to be too much screaming and shouting about the issue. So either it's dead simple and we've missed a trick or nobody else is bothering with sending the parameters in JSON format. I may raise a bug report, for what it's worth.