I have a model that has both fields bound to the original Sitecore template and a couple of custom fields I'm using as part of a conditional. One example is a "NewsUrl" field that checks if a Sitecore link field has a value, and if it doesn't it defaults to the item's URL. When I create my rendering variant and make a field item for NewsUrl, it always comes back with the item's URL regardless.
Another case is a string representing an image path, where I put either the value of a Thumbnail field's URL from Sitecore, or a string that has a static placeholder. This field doesn't render at all in preview mode.
I saw the Model element under the rendering variant and tried setting something there, but it didn't catch and doesn't look like it'd support being part of a hyperlink. So I'm not sure where to go from here to get the rendering variant to recognize the "custom" fields.