The banjo is actually used quite frequently in musical theatre - more often than not, when there is a guitar book, the player also doubles on banjo at some point. A quick perusal of MTI's catalog from the beginning on the alphabet finds banjo used in: Annie, Assassins, Avenue Q, Bonnie and Clyde, The Boy Friend, Bridges/County, Catch Me If You Can, Company, Dirty Rotten Scoundrels... (I think y'all get the point)... and of course it's a staple of the sound and feel of Ragtime.
Cowbell is frequently used as well, though perhaps not prominently featured very often.