There's some info on this link that might help - lists all safe plants - it does say purslane is an unsure one. I sometimes give ours a bit of parsley or a clover flower from the garden grass. Marigolds, cornflowers are ok too and rose petals (although they rarely seem to eat those but go potty for the clover - only white clover though - red clover is too medicinal so could have risks). Dandelion root is safe and good for them in moderation, as is burdock.
I tried giving one of mine some white willow bark when he was elderly and couldn't have metacam due to contraindications - it's a natural source of aspirin and thought it could help with pain relief and couldn't do much harm as he was old and failing. However he just sniffed it and didn't eat it.
Safe flowers and herbs?