mikkie - anything is possible but you might like to ask yourself what is not just possible but highly probable instead. If MOL were to base 737-400s at Southend, there would need to be clear commercial reasons for doing so, not just because they differ from FR's standard 737-800 aircraft, but also because the summer 2014 season that went on sale 5 or 6 months ago begins in less than 3 weeks time, drastically reducing the time for people to buy tickets for any new routes.
Imagine you were in MOL's shoes and ask yourself what you would be thinking about SEN at this point...