I always had some interest in creating such a module, as its very usefule for domain parking.
Let see if we can get the pieces together.
First we need something to lead all domains and maybe subdomains straight to your webspace.
Then we can use websitebaker to sort em out.
As far as i know .htacces can only do redirects(301/302) over (sub)domain boundaries.
so .htaccess is useless for this task.
Ok, Step one.
Something to get all your domains and subdomains straight to your webspace .
On a root server this is done via apache config :
<VirtualHost *:80>
DocumentRoot /web/www.domain.de/incoming
ServerName domain.de
ServerAlias *.domain.de domain2.de *.domain2.de domain3.de *.domain3.de
ErrorLog /web/www.immerbeiderarbeit.de/logs/httpd-error.log
CustomLog /web/www.immerbeiderarbeit.de/logs/httpd-access.log combined
CustomLog /web/www.immerbeiderarbeit.de/logs/httpd-access.log referer
CustomLog /web/www.immerbeiderarbeit.de/logs/httpd-access.log agent
</VirtualHost>
You may notice one interesting thing about subdomains:
Apache accepts wildcards ! So all subdomains go straight to your webspace (
www.domain.de msysub.domain.de abcdef.domain.de all.domain.de..... endless possibilities)
Some, but not all providers offer wildcard support in their maintainance interfaces too. So you need a provider that allows access to apache config or accepts wildcard domains inside of his maintainance interface. If you dont have that, the story ends here , or you have to create all those subdomains manually.

Step two
Now that all Domains and Subdomains go straight to our webspace i see only 1 way of coming closer to our goal
1. modify WB so i displays different content depending on $_SERVER[SERVER_NAME]
Now we need help from some WB experts. Is it possible to modify page ID depending on a Server variable , somwhere early inside of WB ?
I am pretty sure its possible as WB displays different pages as starting page depending on what page is on top inside of pages interface.
Just had the Idea if its possible do abuse the language system for this , so maybe create a language for each extra page and set the Language depending on $_SERVER[SERVER_NAME].
The Language way would have the advantage, that you can have a full scale page instead of just one single landingpage.
Maybe there are several other ways of doing this and maybe DarkViper enlights us how he has done it i would easily take the risk of destroying my server config, as i normally backup all stuff berfore doing any experiments .
Or maybe i simply use my local XAMPP for experimenting.