Since I've just run into a need to embed a form-POST inside a generic object I started reading up on it here. General sense here on the forums is "cannot be done that way". This is due to the fact that ASP.NET really doesn't like multiple FORM tags, especially nested (you should see some of the whaaaa? display issues I was getting). Renders fine in VS2008 without the Master Page but inside SiteFinity, no luck at all.
Some further research and I ran across this article that gives very clear directions how to do a POST with no form required (builds one programmatically). However, you will still need to create a user control (.ASCX) to implement the code-behind.
Use HTTPWebRequest to send POST HTTP Requests to another web server
There is a part 2 if you want to redirect to another address in addition to just sending data. This should give you the ability to have many FORMs (virtually) sending data to many web servers on a single page -and- get it into a SiteFinity control.