Personally I would be delighted to see an ATC bloke/gal on the flight deck. It has to be booked in advance through ops - ie you cannot just approach them in flight and say you are ATC. Nonetheless I know that we do it gladly at easyJet and if I was the skipper you would be welcome any time. I think just about any pilot would have a similar view. The secret is to book it in advance through 'official' channels - ie get your people to talk to our people!
Hope to see you at some stage.