My main concern is still with ease of identification when scanning lists of cards (whether in trades or one's own inventory). Most other card identifiers are easier to identify at a glance.
You are right, this might be a considerable issue. I'm just still apprehensive a bit about adding another user-settable flag for those (not completely sure why... just feels a bit not right). I feel it might lead to people just misusing the flag, marking stuff etched foil when the card does not have such a printing, then adding it to trades, etc...
Regarding proxies, that one is easier, it does indeed need a flag, and special handling for pricing calculation. On the list.