You're right, he's probably at home right now mulling over all the offers he has on the table.....

But how could he refused to sign it?

He could argue that the clause should be removed in order to allow him to behave like a complete idiot and still keep his job?!

I think Dave was making a tongue in cheek reference to when Cooke left Hull to go to Rovers and an 'invisible signature' that he put on his Hull contract that was a joke at the time between some people.
Anyway Cooke as a player would be a good signing, hes the master of the long cut out pass and with Bomber coaching him before and obviously knowing this if you can utilise it it will give you an added extra attacking weapon.