Mr Coggles there is quite a movement among the cigar box guitar fraternity to convert old transistor radios and cassette players into "dirty" sounding amps see
here and as you know, a lot of the volume from an amp has to do with the size of the speaker used, it would be quite inexpensive and dare I say "Green" to go down this route using a relatively small radio. Or you could just hit eBay and shell out a tenner for one of those dinky little personal practice amps that run about 1 watt. You could then fashion an attachment for a hat so that the amp could be clipped on near the ear of the player thereby ensuring that the player hears the sound loudly whilst everyone else in the room struggles to hear what all the fuss is about... :-)