How Can I Mask a Mapped WPMU Subdomain Page URL?

I'm trying to mask a mapped WPMU subdomain page's URL.

Here's a breakdown of what I'm trying to accomplish...

WPMU Primary Domain:
http://a.com

The WPMU subdomain page URL:
http://b.a.com/slug

Same URL with domain mapping:
b.a.com mapped to c.com (1)
result: http://c.com/slug

note: c.com should be a parked domain.

Now mask the page URL: (2)
http://c.com/slug masked with http://d.com
result: http://d.com

note: d.com should be an add-on domain.

IDEA:

(1) Mapping: use the WPMU Dev Domain Mapping Plugin. Simple...

(2) Masking:
- Use RewriteCond and RewriteRule in d.com's add-on directory's httaccess file to rewrite the URL and point it to the directory's index.php

- Use some sort of masking PHP script in the index.php?

I need help with Step #2 for the masking...