How do I use tag questions with indefinite pronouns?

It used to be that the masculine singular served quite well for those who required agreement in the written forms, e.g. 'No one is unhappy, is he?'

With the advent of anti-sexism, however, we have had to make do with such jerry-rigged solutions as 'No one is unhappy, is s/he?'

In the spoken language, the 3rd person plural has long been an accepted match to these pronouns: 'No one is unhappy, are they?' This fact may give impetus to the formal acceptance of the same form into the written language, if coinage does not improve upon 's/he'.