It sure is possible to override and add your own.  I just completed an implementation where i added a bookmark field for "page on this website" where it will fetch bookmarks from the page you select in the pagereference field.  It works well.  what you need to do is create a virtual path map that intercepts the current link editor and it will direct it to yours.  Then you can do whatever you want.

  <virtualPathMappings>
    <add url="~/util/cms/editor/dialogs/HyperlinkProperties.aspx"
      mappedUrl="~/Static/HyperlinkProperties.aspx" />
  </virtualPathMappings>

    I am not sure what the url path is to it but you can see it in firebug where the request is coming from and the mappedurl will take its place and show yours.

Thank you for the response. When you say you created your own, did you basically just modify the HyperlinkProperties.js file although you used the virtual mapping to point to a copy in a different location? I found a few other posts saying this was an option but that it is quite a hefty and inadvisable task.

I realize the work it took to get your bookmark functionality in place so won't ask for your complete code, but even a tiny code snippet to point me in the right direction would be much appreciated. I feel very stuck. Or, if you can confirm it is a modification of the HyperlinkProperties.js file, that's a great start for me as well. I just don't want to go down the wrong route only to have to start over. Thanks again!

if you want to email me.  josh@blendinteractive.com  we can discuss getting your issue resolved.