Hmm, I'd contact whomever decided to schedule him for this shop and tell them that you are uncomfortable taking a shop without any directive, that it would be irresponsible for you, as a shopper, to commit to something without knowing all the parameters and requirements. Additionally, I would let them know that time commitment is a definite factor (as it would be for _any_ shopper) and tell them that without consideration, you would be unable to accept.
Really, I don't know how included you'd be to accept, anyway. Any company that's going to be so inconsiderate as to schedule you for a shop that's obviously time-consuming (and that they are obviously finding it hard to schedule) without your consent doesn't seem to be a someone to align oneself with. Perhaps it's just me! =)
I hope you find a good solution to this situation, or the MSC at least makes it worth your while.
Sending the best,
Jen