Before people get worked up into a lather over a story that hasn't been verified anywhere other than on facebook, Chiltern have got a number of extra trains running from Warwick Parkway. Funnily enough, they're used to dealing with Wembley given that this is on their line so they'll be switched on.
I'd be amazed if there isn't something organised at Coventry, but I reckon it'd be worth getting there early and being prepared to stand up on the way down. London Midland have got at least one extra train - you'd hope that they might consider running eight rather than four carriages for their other services. Virgin run Pendolinos which are nine carraige trains. You can get a lot of people on them, but again it's going to be standing room only I'd suspect.
When I booked from Nuneaton a couple of weeks ago, (by which point we'd sold a large part of the allocation) there were loads of seats still available. I don't think most fans are travelling from there.
I've just checked, and Manchester to Euston trains don't stop at Coventry on Sunday morning, they go via Stafford and Milton Keynes. So no problem on the way down, and Euston always has a police presence in my experience.
All I'm saying here is that we shouldn't create a false panic - but if I was going from Coventry I'd get there in good time.