1. It was he that(who) signed autographs for the fans after the show.
2. It was him that(who) signed autographs for the fans after the show. (acceptable in informal style)
3. It was him signing autographs for the fans after the show.
While I understand the ‘him’ in no.2 above is acceptable in an informal style of a cleft sentence, I’ve heard a few local persons here insist that no.3 is also acceptable, since ‘the subjective relative plus a verb’ could be reduced into ‘-ing’ form.
For the issue - whether this reduced cleft sentence in no.3 as a variant is in fact being accepted by natives recently in informal or conversation style, I would invite your opinions.