I was a bit confused by Max Wyss' answer (sorry, Max!), but found Thom Parker's spot on - when I entered his code...
if(!event.willCommit)
event.change = event.change.toUpperCase();
... in Acrobat XI's Text field properties > Format > Custom keystroke script box (as Thom suggests). All I can add is the 'Custom format script' box remains blank.