How to show custom previews for all fonts in 10.9 Font Book?
This works (at least in Yosemite Public Beta 2):

Open Font Book, select "Custom" from the View menu, double-click in the preview area and type what you want. Then, close Font Book. Then, re-open Font Book. Now, whatever you had previously typed should be the preview text for every font.

Annoying, yes, but it's a lot better than nothing!