Check out this link:
http://www.iceinspace.com.au/63-441-0-0-1-0.html
It's the article by Clive after he moved his mirror.
You really don't want to use a barlow with your DSLR. The focal length of the 12" is already 1500mm native - you don't want to be doing imaging at 3000mm with your DSLR and 2x barlow. It will be much harder, much less forgiving and the learning curve will be much steeper.
I expect you may need an extension tube with eyepieces once you've moved the mirror up the tube but I don't have first hand experience with this.