Hva you checked this plugin out? http://www.tinymce.com/wiki.php/Plugin:advimage
You could use this and modify it after your needs.
Thanks for the suggestion Andreas - but it turned out to be VERY SIMPLE.
For others that may not be able to find the needle in the haystack:
The advanced image dialog is a set doing the following:
Admin Mode -> Page Type tab -> Edit Custom Property Types ... choose XHTML string (>255) in right-hand frame. Under TinyMCE Editor either Edit the existing one or choose Add Setting. In the pop-up dialog, scroll past the Editor toolbar section that shows the available buttons, down to Plugins without a button. Ensure the Change image dialog to Advance image dialog is checked.
Save yourself hours of combing through the haystack!
This is probably reasonably simple but I haven't been able to figure out (or find reference to) a way to update the TinyMCE "image" button so that it shows an advanced dialog rather than the basic dialog. Any pointers?
I can supply images of what I mean if necessary.