Hmm..This definitly seems plausible I guess.. The problem is that its hard to know how much one would need exactly. My hairloss had started before I stopped smoking weed. So i'll never know exactly. In my personal expereince though, I cant say there is that much correlation with the freinds I know who smoke it and their hair..I knew a few guys (all around 20) who are seriously thinning and receeding for their age - a few others who have very very thich hair, so its hard to say.
One guy I know, who is thinning on the front considerablly, is someone who seems is very stressed/depressed. I have definitly had considerable stress in my life - my parents divorcing, a family tragedy, above all, an angry father. So perhaps stress can trigger male pattern baldness. But the same could be said for diet and genes I guess.