As said, it's theoretically right to say to use proprement in the meaning of d'une manière qui leur est propre. The thing is that in practice I've never heard anyone using this adverb to emphasize an adjective, not even misused. People tend to misuse littéralement sometimes, although less often than the English with litteraly. If you used proprement in that context, you would sound as if translating litteraly from English to French.
Actually the main reason for it is that the first meaning of propre is clean. Proprement means on a clean way like 'Mange proprement' as a parent would say to his child.
Though in some widespread expressions as 'Proprement dit' or 'À proprement parler', it doesn't mean clean at all, but rather properly.