Full fenders with a LONG mudflap on the front will give you 80% of the benefit of a chaincase as far as keeping crud off the drivetrain and infinity% of the benefit of a chaincase as far as keeping your feet dry. One is still susceptible to the dreaded chainring tattoo though.

Wasn't there a downhill MTB made with a gearbox in the frame, which, when opened, contained a regular derailleur system?