I have done the Foreigner twice.  I played Charlie both times and designed both sets.  In one set I was fortunate enough to have an actual trap door for the cellar entrance.  Both cellar entrances were basically boxes about 2 foot by 2 foot with a hinged lid on top.  The problem lies in making a quick exit down.  This ended up being impossible.  I solved this by creating a spring loaded panel in the wall above the cellar entrance.  Charlie would set off a piece of flash paper in the blackout and simply push out the false wall.  Once out back stage I just crawled under the back of the set and appeared to come up from the cellar.