We're removing the render method from CustomDatatypeHandler, because it doesn't belong on that interface. But the idea is important.
We should introduce a new interface in the web layer that (maybe) extends CustomDatatypeHandler, and (definitely) is extended by both FieldGenDatatypeHandler and WebDatatypeHandler. My initial proposal is something like this:
I'm thinking that code which displays these custom values in summary pages would call the toHtmlSummary method, and if the returned summary is not complete, it can display a link to the full view of this value. But the handler implementation doesn't need to worry about including the link in this common case.