I have implemented a form of the "preview-set" property. It looks like this:
<mime-type>a MIME type</mime-type>
<width>number of pixels</width>
<height>number of pixels</height>
By including all the information in the property it can simply be included in a PROPFIND of a complete collection. The server can return several previews for a resource, with different sizes for example. Depending on what is desired by the client some URL can be used to fetch the actual preview.