I've hennaed over previously chemically colored hair and it turned out just fine. Be sure to use BAQ (body art quality) henna; you don't want to use most store brought henna because a lot of it isn't pure which could be troublesome for chemically dyed hair.
APL BSL MBL WL HL TBL | porous, fine/medium strands, medium density
My Fotki!
Below: Typical PS bun