hee on the av. and I've done all of the methods mentioned for washing hair with very little water when I've camped. I've also used witch hazel on the scalp in lieu of washing it (when I had hair extensions). Those methods have worked fine for me. For reference, I have shoulder length, medium weight hair that is prone to knotting.

I have not needed to do an entirely water-free hair wash so I don't have first hand experience on the white vinegar or lemon juice. However, given the choice I'd use white vinegar. Because it's not as sticky as lemon juice, is more readily available, and doesn't require refrigeration. It also is less likely to attract bugs. But when I was a kid we used to spray lemon juice in our hair to try to sun bleach it so we'd have summer streaks.

The only time I think I'd use the tea tree dilution was if I was feel seriously grubby or if lice were a concern and I was trying to do a preventative treatment. Or if none of the other products were available.

I personally wouldn't use rubbing alcohol on my scalp or hair because it dries out both, which could result in an itchy scalp and totally unmanageable or breaking hair (which is why some people try to avoid styling products with high alcohol content). Witch hazel is less harsh. However, I have combo skin that tends toward dryness and dry hair. Someone with oily skin or hair may find alcohol works well.

I've read about baby powder and also thought it sounded like a mess. It might be ok if you have really short hair.

Also, keep in mind there's a difference between corn starch and talcum powder. People tend to use the term "baby powder" as a generic catchall for either. I'd opt for corn starch there over the talc. I don't know if either is biodegradable, but I'd venture that corn starch is better than the talc in that regard.

hope that helps

oh...
I wonder if hand sanitizer would work for the scalp or hair, keeping in mind my comment about the drying effects of alcohol earlier.