Why is ElGamalEngine in bouncy castle restricting input data to be less than the length of the group parameter p?
Originally we had restrictions like this in place as the alternative was emails about bugs in ciphers like RSA resulting from people not understanding the implications of modular arithmetic (if you are going to push right to the boundary you really have to understand what's going on). Having said that, I actually think we'd outgrown this by the time ElGamal appeared (around BC 1.12).

Oh dear, looks like there's another test further down. Perhaps partially outgrown would be a better way of putting this.

Okay, this is now fixed and checked in it should appear on github shortly.