Hi Tomas,
What I did previously was to inject a Javascript function using the "Insert Javascript Fragment" action and subsequently used "Invoke Javascript Function" action to execute the same function. The JS function should create an element on the web page, preferrably hidden, and then store the value against the element. Once done, spy the hidden element, and then you will be able to use a simple Read stage to get the value from the element.
I have not tried anything simplier than this, but this does not mean it is not possible. Hope this helps.