I think that previews for betas would serve not to turn people away, but would draw more testers in. Providing a demo for published games is highly effective; you gain readers who like what they see and want to continue. This could work much the same way for betas. If someone is really excited by a preview, they’ll be highly motivated to apply to continue. I know when I was starting out on the forum, I wouldn’t really find official betas to grab my interest, because all I’d have to go off of was a premise. Seeing what the writing is like has a much bigger impact. I made a similar suggestion close to a year ago. I’ve talked to other people on the forum who’ve felt similarly reluctant. These people are excellent testers, and have shown so in WiPs throughout the forum, but it feels like a big commitment to apply to a beta when you have no clear idea at all whether you’ll actually be suited to it. This doesn’t mean that beta testing isn’t for them. They may have a lot to offer, but without getting a glimpse of what a game might look like, they may never even notice a beta that they would really be able to help.
Closed betas are certainly useful, but it’s worth noting that many of the most successful betas on the forum started out open before becoming closed. They were able to attract a variety of highly interested testers by showing them a sneak peek.
This could work just by showing a beta link in the post recruiting testers, and having the opening of the beta be visible, while everything past the preview is password-protected.