Rosebay Willowherb is a funny one, it appears to be safe for buns (we feed it ours with no ill effect) however it's toxic to horses and cattle. Goats and sheep have a resistance to pyrrolizidine alkaloids found in willowherb and the rabbits digestive system is very similar to this group (they are known as Pseudo-ruminants, i.e "hind-gut fermenters")
Because it's a bit of an unknown I wouldn't like to recommend it (however we've never had any problems with our buns, and they love it)
There's a good article discussing lethal dose here
http://www.comfreycentral.com/research/pa.htm
You'll notice from the chart of comparative ranking of animals based on susceptibility to poisoning, the rabbit would have to be fed 115% of it's bodyweight for a lethal dose compared to just 4% for a cow. Many, many plants however contain PA's (even ones on documented safe lists).
Nettles are fine, i tend to dry them in an oven on low heat for a couple of hours as it removes the sting.
EDIT, on a completely off bunny note, you definatley shouldn't allow horses to eat RBW.