I can't offer any advice specifically about sharing things between asp.net and classic asp but I think doing some google research you may find a way.
However I know that you can protect static files in a given folder beneath your mojoPortal site by adding a web.config file to the folder like this:
<?xml version="1.0" encoding="utf-8"?>
<allow roles="Admins" />
which would allow only users in the Admins role to access the files. The same solution "might" work for protecting .asp files in a folder, I'm not sure though.
Hope that helps,