It helps that Song is probably perfect for it and can play in the centre of defence anyway, but if I went another team and used this tactic I'd probably still use a DM to play the role.
My system is based on Biesla's "spare man at the back" theory. If a team plays 4-4-2, I'll counter it with the 3-4-3 - both centre halfs man mark their opposite number and Song is the free man. If they change to 4-2-3-1 then Song comes out of the back line into a DM role and man marks the opposition's AM, and Vertonghen becomes the free man at the back. I'm considering developing a 4-2-1-3 tactic for use at home against pish opposition, that would line up something like this:
Isla Koscielny Vertonghen Santos
Walcott van Persie Muniain
Started off quite well so far; have beaten Stoke 3-0 away, Udinese 3-0 at home and Chelsea 1-0 at home (Cech was man of the match with a 9.3 performance, and the woodwork was probably Chelsea's 2nd best player). As in my previous Clyde game, I'm finding that my team is pretty wasteful with chances (had a 2-0 preseason win against River Plate that featured almost 30 shots on goal with something like 5 on target.)
edit: the other advantage of playing Song in defence is that if the opposition changes tactics during the game I don't have to waste a substitution to counter it.