You should be fine - just make sure it doesn't dry in your hair - wrap it completely in cling wrap and a shower cap or something.
So... I tried henna without honey. I just rinsed it this morning. So far, I detect no difference in moisture or softness. This may be because this is my third henna and my hair is already well-conditioned. Rinsing wasn't difficult, per se, but I think I got more of the residual dye out faster when I used honey. I'll have to continue experimenting.
I've decided that henna is the best deep conditioner money can buy. I bet cassia is right up there with it (I haven't used cassia on my whole head yet). After this last henna, my hair is seriously softer and in better condition than it's ever been in my life
! I'll knock on wood that it keeps working that way. Regular deep conditioners seem like temporary fixes to me now - only good until the next wash.