Do you want to actually edit the value entered, or just format it in this way?
I think formatting is all I need...
Or value entered... I don't think it matters much.
I suggest using formatting only, otherwise you're going to get problems with the script running multiple times and inserting multiple spaces. Use this code as the field's custom format script:
var letters = event.value.split("");
for (var i=letters.length-1; i>0; i--)
event.value = letters.join("");
Nice one. Exactly what I was after. Thanks so much for your help.