redirect from www.mysite.com to sub.mysite.com

But there is a BIG catch so i need your help !

[nickg]

Background
I'm the SEO guy for an internet based company verybigsite.com which has about 1000 employees and only gets revenue from the online activities going on the home page of this site.
Amazingly there is no visible text on its front page, yet because of the meta tags & backlinks it does really well on some BIG ticket KW's

So...because we run a very delicate system there has been a policy of setting up sub domains for all the peripheral sites surrounding this sumo sized site, so the golden goose does'nt get broken by anyone

Perameters:
- I can't have megasite.com/mylittle_subfolder/ but I can have subdomain.megasite.com/my_content/
- I can have
> the only clean link from the front page of megasite.com to subdomain.megasite.com/my_content/
> I can set up a robots.txt file
> I can set up an alternate link i.e. <LINK rel="Start" title="Go to place with readable content" type="text/html" href="http://subdomain.megasite.com/my_content/">
- And I can clear out all the meta tags from megasite.com, making it a completely white sheet for the engines (apart from the above)
- By the way all the inbound links run into megasite.com/

I guess I'm thinking of a cloaking synario i.e. home page for users and subdomain for google > then users
or
a pipe running from the front page through to my sub domain that google runs straight through - almost like 301 rdirect without the users who land on megasite.com/ being redirected the subdomain

So the main question is:
- how do I get google and others to rank my humble subdomain as though it were megasite.com (the place where the backlinks are going to).
- Would a combination of an alternate link, clean visible link, robot.txt doing some clever stuff do the job?

The other synario is where I tag the front page up, discretly add 1 sentence with my big ticket KW's and a clean link to my subdomain in a search visible part of the site.

Any good ideas?