I'm not big into Golden Age spy comics.

Could be a super-hero comic as well. She said that she was pretty sure Captain America had a decoder ring, but that book wasn't around until the 60's.

I only remember them as giveaways to fans.

In theory, the giveaway would be a product tie-in, something they used in the book as well.

Does it need to be the main character that users the ring or can it be a subordinate (that is, Kato instead of the Green Hornet, not that that example fits)?

Yes, I believe that would be fine.

Can it be a strip or does it need to be a book?

I believe it needs to be a book.