Hi,
I'm a CAD programmer for over 25 years. I created programs for AutoCAD using Autolisp, VBA and VB .Net.
First, it is not possible to recreate exactly the same open command using Autolisp. We have to use .Net (VB or C#) or ARX, maybe VBA but I don't think it will be the best way. I already created similar program for my product (Le Classeur À Plans), it is a drawing manager use to start AutoCAD and open directly the drawing if the drawing is already in use, I show a dialog box with user information. This was required because the drawing is open from an application running outside AutoCAD.
When you use the Open command, AutoCAD already show you the user information if the drawing is already open. I don't understand why you want another command. To replicate exactly the same open command, it is a lot of work and I think useless.
Feel free to contact me for more information.
Regards,
André