functioning as an adjective

Doesn't seem like an adjective to me. Seems like an old-fashioned compound noun to me. For example:

1. Pick up your lego bricks and put them away.
2. *These bricks a very lego.
3. *The older, primary-colored bricks are the most lego of all.

I have never heard anybody say anything but legos when referring to lego bricks. It's sounds as unnatural as saying "that is consistent with hoi polloi".

I do pity the poor folk who actually use the language though. If they shorten phrases, such as ice cream for iced cream or lengthen them, such as ATM machine or PIN number they are damned.

Ceci n'est pas un seing.