I am seriously considering biting the bullet and replacing the Xinha HTMLEditor with CKEditor. It seems the CKEditor folks are working on making their editor play nice with jQuery.

Found list of links on the topic of building plugins for CKEditor:

http://stackoverflow.com/questions/1957156/ckeditor-how-to-add-a-custom-button-to-the-toolbar-that-calls-a-javascript-funct/1966239#1966239

If I do this I'll need to redevelop the linker and imagemanager plugins.

Another option might be the TinyMCE editor which, unlike CKEditor, has a large number of existing plugins.