Tom,
As many here will know I'm currently putting together the plans to build a 12-string Parlour guitar so I have been talking to a number of luthiers over here. They all say that a bolt on mortice and tenon is quite man enough for the job but that I should also incorporate a couple of carbon fibre rods in the neck to increase stiffness and that head block extensions are a good idea. One area that you should obviously take great care of is top bracing, it needs to be man enough to take the additional tension. This isn't such a problem in fact with the Parlour body size I'm using but for anything bigger, and I assume you're not nuts like me so will be using a Jumbo or D size body. The luthier over here that I respect most uses AXX bracing on all his 12 strings replacing the tone bars with an additional X brace. Taking care obviously on its position relative to the bridge, as has been rehearsed in a separate discussion here.
Also consider a slot head, this is what I'll be using as many people pointed out that a solid head 12 string can get a bit heavy (don't forget you can use mini tuners.)
Colin
