I've tried to rinse my dreads with hibiscus and rosehip tea just to see if it would make any difference and well, I wouldn't want to leave it in the hair tbh without rinsing. I've noticed that if I let it dry with the tea still in the dreads the ends are like glued together so not sure I would just want to let it sit there without properly rinsing. However I imagine that it gives the hair a nice shine but not much difference in the colour.
If you want some more red tones in your hair why not just do another henna? Henna will build up in colour but it can't make it lighter so for example on dark brown hair it won't go copper - but will give it more red shades and reflects. Make a strand test if possible tho before you go ahead:)