I left my referenced masters elsewhere and would like to export the preview itself for sharing online. How can I grab a copy of it?
I suppose taking screenshot is one possibility, but would rather export or access the file directly.
Last time I used Aperture, I went into my iPhoto/Aperture Library file. It appears as a package/single-file in Finder, but it's actually just a folder. Control-click or use Right-Click on a two button mouse (a.k.a. "Secondary Click," many ways to use this) and select "Show Package Contents. Through here you will be able to navigate to various folders with previews, older versions, etc. etc. You could do a finder search once inside and narrow it down by size and name and search scope to just that folder. should be quite small. less than 50k methinks, if not a fraction of that.
This will vary quite a bit depending on what version of Aperture/iPhoto you've been using, if they are "Sharing" a library, etc.
Last version of Aperture I used was 2, and I had the library separate from my iPhoto, but I know the latest versions now share the Library File, in which case it should probably be in your "HOME" folder, in the "Pictures" Folder.