Sanford Whiteman is this still an unresolved issue? I am in dire need of a solution for this problem right now.
I should add that I only need to be able to do this for existing leads.
Never mind, I think I found what I needed. In "visited web page" there is a "querystring" constraint.
Actually this was the other Sanford Whiteman's question ( ! ) but my answer would be to perform a hidden form post if you want to update a custom field on the lead. There's nothing that stops you from using the forms API in this way.