I think the problem is that while you are sending the request as an HTTP POST, the YQL query and text are all included in the URL. YQL does not really have a way for you to make HTTP POST requests directly, so I can think of a couple options:
- Directly use the Content Analysis web service with an HTTP POST (docs)
- Create a custom YQL data table which uses the
<execute>
tag to run custom JavaScript which could do the POST (example)
Of these options I think the former would be easier.