Speak to
www.LTpaintball.co.uk, they might be able to help you out with annodising. Also Smartparts Europe might be able to help, give tham a call.
I have, however, heard of a few horror stories on the US forums about re-annoing Shockers because of the type (read: quality) of the metal used.
The annodising itself is not the problem, its the stripping of the old annodising could cause pitting and such like.
Not sure if the stories were true or not, so don't quote me on that, but I would do some homework first.