If I did a search for some product I wanted, ended up on your landing page, and discovered that you currently had nothing to solve my problem, I'd consider it to be a waste of my time. I wouldn't call it "scummy" though, as long as the landing page was honest about the product not actually existing yet.

I wouldn't sign up for a wait list unless there was a clear time frame for when to expect a working product. If I have a problem I need to solve today, being able to have your MVP product six months from now doesn't really help me.

There's nothing dishonest about a waitlist for a product that no one can use yet. As long as you don't lie about other things -- like saying "x,000 users!" -- then you're fine.