can .webloc file icon show a useful web page preview?
I did finally locate an application that provides the solution you are looking for.  The application isn't hands-free initially as it is a command line program.  

setWeblocThumb generates custom icon consisting of a preview of the web page that the webloc file points to.  It does not do this universally.  You'll need to 


point it at folders that you'd like it to watch
Assign a folder action to the folder that you'd like setWeblocThumb to watch and have it run setWeblocThumb.  


It's fairly straightforward, but is a bit more than simply downloading and installing an application.  The good news is that it's free.