@Muarck,
Hmm I imagine at a certain point temporary tips into permanent. Sounds like what you're looking for is something that won't wash off in the shower, not until several washings have been performed (say, 20? 25?). Kind of like semi-permanent hair coloring.
I wonder if you could use semi-perm hair coloring? I am thinking you'd never get over the hump of getting past skin irritations or allergies, though. Wish I could be of more help -- is henna on skin fairly durable?