You could use hashing... (Score 1)

You don't necessarily need the actual text of the original code in order to provide context. In principle you could use some sort of sophisticated hashing scheme to give context without giving away the actual text.

It's boils down to something like replacing "put '++i;' after 'if (p)'" to "put '++i;' after the line whose hash is 06A204EF". A proper hashing algorithm, like MD5, can give you both a very high certainty (as in, one in 2^128 or so) that you're patching the right line, but give away nothing about the original text (which could still be one of 2^infinity possibilities, any of which would be computantially impratical to find).

The interesting question is whether an MD5 of a portion of GPL code is considered itself GPL...
It's the kind of question that makes me want to just forget the whole issue and go to sleep.