Weller would look like the biggest hypocrite ever if they did a full gig - at best I could see them maybe doing a couple of songs, and even then I would imagine there'll be other musicians on stage as well (Noel Gallagher and Steve Craddock being the likeliest).
They have marketed this charity gig very cleverly to make it look like he is performing, but with the ability to say "we didn't say he was", should he decide not to. I think the only reason people even think he might be involved is because his sister has organised it.