I think they are not THAT rare... there was one on Jamma+ 2 months ago for 550£ which is a really nice price if there are really only 10 available in the whole country.
Regarding the marquee signing, I had the same thought as level42 and I think I'd like it better on the sideart...but then it's much more difficult to send by mail

Anyway, it has to be on a part of the cab, much more interesting than on a piece of paper imho.
In this case I was thinking that maybe if you're trying to break a world record, it must be great to have a little word of your "mentor" just above your head so you can easily read it before each try to encourage you.
Tony's version of MC is slightly customised too (tainted glass, glass marquee, special writing on the coin stickers) so I guess, he doesn't really care for sticking as close as possible to the original cab as we do. He just want a nice looking and effective machine.