I don't know whether this has any real revrence to this, but I just feel I need to address it.
Firstly, you're saying this is a B.E. How to Draw. Are you just focusing on B.E. as a whole or just one fundamental aspect. B.E. actually covers four general parts, the 4 B's as they're called: Breasts, Butt, Belly, and Body. If this is going to be a real How to Draw B.E. you need to be taking that into account. For reference check out:
http://groups.yahoo.com/group/The_Four_Bs/
Secondly, reguardless whether or not you go the full B.E. course, I think you need to include a little bit on the different methods through which the expansion can occur. You know, magic, vore, nature, inpregnation, inflation, absorbtion, injection, machanical, fat, etc. because sooner or later a young artist is going to ask for some details on how one or more of them affect the ways, looks, and action of the expansion. Some are fast, some slow, some more active, some not, and some who knows whatever. It's just something you should consider nipping in the bud early and it can add a bit of fun education.
Thirdly, if you do go the long all with all and cover B.E. as a whole, you'll need to include all spects you listed earlier for each B. You know, the effects of gravity, on clothing, etc.
Fourth, I'd suggest not only covering humans but also the humanoid frame in general. Because furry artists are also going to be interested in this and the rookies will want some help in understanding the affects each type of B.E. has en regards to the animal features. Not to extend this into and encyclopedia or nothin'; just be sure to cover the basic affects each has on the humanoid body and how it they might affect the assessory features, such as wings, tails, ears, alter-legs, fur, pouches, etc. It's something I think you should keep in mind.
And lastly, and this for the reel newbies, think about putting in some words or thoughts in the opening to help some of the younger or troubled artist interested in this to get over their moral blocks. 'Cause I doubt I'm the only one who grew up with strong moral and church teachings that went far more than just right and wrong. If they're anything like me, they'll be having trouble not just with drawing skill by psychological blocks that trigger everytime they try to draw something heavier than Rated-G. And it'll probably help to here some tricks or thoughts from a real artist to help them over that kind of "pain-in-ass" delima.
Just some thoughts and suggestions I had on the subject that I thought should be addressed. Hope they're helpful, and I'll be waiting to see what developes.
I'm always looking for new art lessons and books to add to my little library.