Ive been looking for this myself. And found a way to do this...probably not according to DNN-rules, but hey? it works !

have your IFrame directed at an aspx file that you've uploaded in the rootfolder of your DNN site.
You add the userID and PortalID in the query string.
In the code-behind of the aspx file you wirte in pageLoad:

dim thisUser as New UserInfo()
dim portalID as integer = val(request.querystring("PortalId"))
dim UserID as integer = val(request.querystring("UserId"))
dim CurrentPassword as string
dim CurrentName as string = user.identity.name

ThisUser = UserController.GetUser(PortalID, UserID, False)
CurrentPassword = UserController.GetPassword(ThisUser, "")

response.redirect("http://my.remotewebsite.com/othersite.aspx?username=currentName&password=currentPassword")

good luck!