I've come across this same problem, and I've come up with an admittedly hacky solution to it.

When a node is previewed, I take the path and check to see if it can be parsed as an integer, as the URL is of the structure "http://yoursite.com/1234.aspx". If the first part is an integer, I treat it as a preview item.

The key part is the call to GetById.. One of the overload signatures has a bool property that allows you to state if you want the preview item. Set this to true, and you get the preview item as IPublishedContent.

var path = contentRequest
    .Uri
    .GetAbsolutePathDecoded();

var parts = path.Split(new[] {'/'}, StringSplitOptions.RemoveEmptyEntries);

int nodeId;

if(parts.Length == 1 && int.TryParse(parts[0], out nodeId))
{
    var contentCache = UmbracoContext
        .Current
        .ContentCache;

    var content = contentCache.GetById(true, nodeId);

    contentRequest.PublishedContent = content;

    return true;
}

As I said above, I don't feel that this is as clean a solution as it could be, so I'd love to hear if there is a better way of doing this.

You could also preview all the content of the website. Than the IContentFinder should not return a 404. See this example: https://our.umbraco.org/forum/extending-umbraco-and-using-the-api/78849-preview-all-content#comment-252857

Jeroen

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