With a Static Mashup, just put a Panel over the others ( z-index ) and with a Refresh widget query a property that records lastActivity, if lastActivity it's over 5 minutes then turn panel visibility on. The panel should have a big button ( or valuedisplay ) in order to be clicked and hidden.
The lastActivity property should be on User Preferences ( in order to have more than one screen for different users ) and should be set anytime you call a service from a mashup, you can do it on the service itself or on the mashup on ServiceInvokeComplete event of all the services called on the mashup.
