I don't think I would ever do it, but given that:

i) the BP doesn't have to state your status. For example, you may have BA status and choose to credit some flights to Finnair to top up your AY+ account before your AY miles expire. You can enter the lounge based on your BA status, but the BP won't show that,

and that

ii) the vast majority of the lounges I've visited have been unable to read the magnetic stripes on a non-native card. In fact, the dragons quite often have to check up their "cheatsheets" in order to see which cards can be used for entry,

I imagine that it would be pretty easy to forge a FF card of an airline that you never fly (e.g. LA if you're a shorthaul BA flyer) and convince the dragons that their computer's dodgy and doesn't display correct status. Plus, should you fail, they won't confiscate your card (can you imagine them saying "this card is property of an airline based 5000 miles away whose planes I've never seen and who doesn't even fly here and we'll return it to SCL just so you have to wait for three months for a replacement?" ) ...