Nope, it's all already OGC. By my understanding, it should be no different than redistributing the SRD -- or, more properly, transcribing it. I think there would be some issues with just scanning or OCRing it. IANAL
That said, I'd love to get an electronic version of UA, too. I've got the hardcover, but it'd be easier to cut and paste the bits I use into a smaller reference document.
You don't have to add anything to open content when you use it, though it's nice when people do.
You might be thinking of a provision in the d20 license that says at least 5% of the text of a product must be open content if you want to use the d20 logo (this keeps people from slapping the logo on their book but then closing everything inside). But there's nothing that says any part of the open content must be new - it can all be stuff that already is open.
Yes, and that exactly what many people have done. Mongoose's "pocket player's book" comes to mind, as do the various PDFs of the SRD you can buy from RPGNow.