As you already know by now, you cannot use the DAP Wizard to set or create
an Input Mask for a DAP page; so you will have to create your own piece of
code using either Javascript or VBScript. This is not an impossible task
but it will require a very good knowledge of Javascript (or VBScript) and
DHTML.
For an example of script, see
http://www.codeproject.com/jscript/Form_field_input_mask.asp or search
Google with something like « Javascript Input Mask »
Also, even with the correct piece of code, you'll still have to integrate it
inside your DAP page; which is something that won't be easy.
There is a newsgroup about DAP: m.p.access.dataaccess.pages
If you don't have a direct access to this DAP newsgroup with your current
account, then you can access it through Google:
http://groups.google.com/group/microsoft.public.access.dataaccess.pages