You should try to post on the Comments collection:
ugcCall.UploadString("/Comments", "POST", ugcData);
Then you will see that you're missing the CreationDate, moment in which you need to add to your entity something like:
\"CreationDate\":\"/Date(1359457694472)\"
(I have not actually checked if you need more quotes in there). For the format of the date in a JSON string check the odata specs.
If you still have problems, try to change DOMAIN%5Cbsmith
to another dummy value ('test
' for example).
If that is not enough then maybe you can look at the logs generated by the UGC WebService and try to make-out some stack-trace.
One more thing to notice here: the UGC properties need to be defined correctly in the Web.config in order for the post to even happen.
Hope this helps.