Would seem less complicated if he was just to make his own custom dialog box and call it to open in the JS of his own website. With the WebScheduleInfo dialog opening event. oDialogUrl("YourUrlHere.aspx") or whatever the function is.

And he could simply make a session variable that carried over from the logon that had a true or false in it. Then when creating his Form for add appointment he could place the admin material in a panel and hide that panel unless session["isAdmin"] = true. 

This way you aren't doing anything extra complicated and you can set the dialog box to do everything you need to do.

1. Specify the JavaScriptFileName property where scripts were to be loaded from on the WebScheduleInfo (so that your modified JS is used instead of the embedded JS resource that gets used by default), and

2. Re-make any modifications to the script after updating to a new service release, or upgrading.

If your user logs in as either a normal user or a system manager such that they will always see the same dialog, then you can just change the WebScheduleInfo AppointmentFormPath property.

If you sometimes need to present a classic dialog, and other times a management dialog, to the same user based on dynamic conditions on the client, then there is more logic to implement but I think it should also be possible.  The AppointmentFormPath is used to formulate a default URL at which the dialog HTML window is opened, I'd think you would need to switch that on the fly as the conditions change.

You can copy over the existing WebSchedule Forms into a different subfolder, and modify them to create the management dialog.  "Including WebSchedule Dialog Forms in your C# Web Site" describes all of the files from the built-in forms you may want to take into account (things like the mclistbox.js you can probably share, dialog-specific files you could customize).