I agree with Reb Eliezer.
I don’t have an answer to the OP. You’re right, it’s a halacha shaila, so I don’t want to give my “conversational” opinion only to find out it’s against halacha. I know there’s a halacha in hilchos lashon hora that if someone is in a store about to buy something, you can’t tell them “It’s cheaper in the other store” (assuming the price difference isn’t אונאה). I don’t know how that applies to this case though, where you’re offering it for free.
Fascinating question.