I think it's quantity that might be your downfall.
Y'see, we can eat apple pips, and not kill ourselves, BUT, don't eat too many of them.
The apocryphyl story goes that a prisoner who was given an apple a day saved the seeds, and when he had an eggcup full, he ate them, and killed himself with cyanide poisoning.
We know that there are similar things in rosehip seeds, but, the seeds are used as painkillers for arthritis, and appear effective.
This is a link to Eat the Weeds, which is generally pretty sound
There is an article on the very seeds, and loads of links that are worth following and reading.
Some species of Rosaceae family do contain a small amount of cyanide. However, I can find no reference in any scientific journals stating that rose hip (Rosa canina) seeds are toxic.
www.eatweeds.co.uk
Personally, having eaten and prepped all sorts of things with rosehips over the years, I find the wee hairs are not easy to remove.
Even well made rosehip syrup, hold it up to the light, and you'll often see those wee hairs floating, almost invisible, but there in the syrup.
I know, I'm fussy, so these days if I want rosehip flesh, I use a sharp knife and I peel them, thus avoiding the digestive track irritant that is the hairs. You can freeze the flesh afterward to mimic frost if desired.
As for the seeds, I think it's the rabbit starvation thing again. The effort involved, and the amount you'd need to eat, and mind we don't have a digestive system that will manage those without cracking them, or be able to digest all the nutrient that is technically available. So, energy output vs that gained.....hmmm.
Me ? I'd add it into the list of things I can pick and munch as I wander, but I wouldn't make a meal of it.
M