Hi,
If you go with a regular button with the following javascript in the click event:
app.execMenuItem("SaveAs");
This will open a dialog which allows the user to save the form (provided they are using Acrobat or you have Reader Enabled the form before deployment).
You can give the button a caption "Save for submission" or something similar.
In addition you could script a popup dialog box giving the user instructions about how to send you the completed/saved form. But modal message boxes can get annoying for experienced users of the form. Maybe some static text in small font near the button or use the tooltip to provide this information.
Good luck,
Niall