In my opinion overlays are always a problem. For example I removed the registration overlay on my site and redirected to its embedded page.
It gives a "pop-up" feeling. Something jumps in your eyes. You can always accidently close them. If you have to scroll them, are you supposed to scroll them or the page behind them? A normal user doesn't know how to link to them, if they wanted to do so.
I see its advatange, as it doesn't redirect you to a new page, when you have an overlay, but I try to avoid them if possible.
For your situation, I would make my decision based on a mobile phone. How does it act on there? If you think it feels not weird, go for it.