Jet2 B757s do visit on the odd occassion, but I think maintenance related. As mentioned the Jet2 737-300 (in Channex livery) does mail flights at night and occassional charters other times. Have heard that Jet2 may be looking at STN for B757 flights in future, but no other info.
Not aware that Excel 738-s are being used by Ryanair, last time this happened was March 2005. Earlier this year they used Futura 737-8s and 737-4s and occasionally Eirjet when their own crews/aircraft not available.