Robin, Stian:
I think I agree with Robin's last message. Let's summarize:
If what you want to do is to specify, within a portlet, the initial XML data Orbeon Forms should use for a given form, you CANNOT use session/request data. It just won't work, at least not without further changes to the proxy portlet. I have in mind the following.
Form Runner supports POSTing initial XML data to the /new page:
https://github.com/orbeon/orbeon-forms/wiki/Form-Runner-~-Configuration-properties#initial-instance-posted-to-the-new-form-page(see "Initial instance posted to the New Form page")
So you could modify the proxy portlet to do a POST of this data to Form Runner, from the portlet. And Form Runner will respond with the HTML to embed in the portlet, as usual.
So:
1. Proxy portlet obtains XML (which yes, it could obtain from the portlet request or the portlet session).
2. Proxy portlet POSTs that XML to Form Runner on the other side.
3. Form Runner response contains the form initialized with that data.
But this is currently not an existing feature, so it would need to be implemented.
-Erik