Hey, sorry to reply to such an old thread, but I had some thoughts on grafts with woody stems and the potential effects of grafting a small shrubby indica onto a heavy root producing sativa.

I imagine the most foolproof method of grafting woody stems would be another bonsai technique I've seen a lot of success with. It's called approach grafting, and it's only slightly more technical than the way that trees graft themselves in nature. Basically the way it works is take two rooted plants, shave some bark off the stems where you want them to graft, and then tie the wounded stems together. This way your scion is getting everything it needs from its own root system, and like air layering vs. cloning, it should have a pretty high success rate. here's a link: http://aggie-horticulture.tamu.edu/p.../approach.html
after the graft takes, you can then remove the top of the rootstock and the roots of the scion... or just have a double rooted, double topped plant, if that's what your going for.

As far as the idea of an indica on sativa rootstock, I get the feeling that the indica top might grow faster and more vigorous. One of the primary reasons that fruit trees are grafted at the base is to use the roots of a tree that's been bred to stay small. This restricts the growth of the scion and usually keeps them small and manageable. Apple trees grown from seed can get 50+ feet tall, but when you graft one to a dwarfing rootstock they top out at maybe 30 feet in 15-20 years.
Now I'm not sure that this would work the other way. The indica might just grow at its normal rate and you could end up with a weird, bulgy graft union.

Anyway, I'd love to see an update to this thread if any more work has been done, and I'd be really curious how flowering would go with a grafted plant.

Thanks for reading, and keep up the innovative work!