I wouldn't recommend it either. (I just henna'd my hair after I took it out of smallish-medium twists...)

It's not hard to rinse out (depending on your brand...and ability to filter), but it takes a while to completely rinse out. I don't know if you'd be able to completely rinse henna that gets caught in twists.
Natural since July 2009
Hair Type: 4a
Regimen: Ayuverdic + Natural
Blog: A Naturally Kinky-Curvy Cyster