Here's a list compiled by Dennis Baron of all the proposals that have been made over the last hundred years or so for a pronoun to serve this purpose. You'll notice none of them have caught on. (My favourite is h'orsh'it.)
http://www.english.illinois.edu/-people-/faculty/debaron/essays/epicene.htm
Between you and me, the answer to this conundrum is undoubtedly to say 'they', which everyone does anyway. If anyone tells you this is hard to understand or leads to terrible ambiguity, point out that no-one has had trouble with the plural/singular 'you' since 'thou' died out.