Hi,
Here is what I would do. It might be a bit complex but here it is.
In the doctype definition I would create a 3rd field that would be assigned to the Document Name
At this point you have multiple choices...
1. Use a custom form layout (in the form tab) add some javascript to populate the 3rd field.
2. create a custom Form control and code the some javascript here (lots of exemples in /CMSFormControls (
CMSFormControls Doc)
3. Create a custom New and Edit Page for Doctype creation (here you can handle any code/validation) you can use the API to create your document.
Francois