i would use the plesk software, which means, that i would create the different domains in plesk linked to the same folder. To do this u have to edit the vhost.conf for each domain (or subdomain) in Plesk to fit your needs.
Hint: You said that you want to edit the httpd.include File. I am not sure but so far as i know this will not work. Because it will be overwritten by plesk from time to time and delete your entrys. You have to do it like i said, on the vhost.config. Maybe this depends on which Plesk Version you use. On quick search i found this: http://download1.parallels.com/Plesk/Plesk7.1/Doc/html/plesk-7-reloaded-admin/apas02.html
I have a little bit experience on plesk ( i stopped using it), maybe later, you want to know how to disable safe_mode and so on, then just ask again.
We have used Plesk before in the manner you describe.
The setup of using Plesk with a default eZ Publish install pre configured works well enough with some effort.
When I accomplished this task, it took quite a bit of
configuration changes global and local to the default configuration of the vhost template rules per site.
The real plus of using Plesk in the end is the ability to simply
automate turning up a fresh copy of a default or customized build of eZ Publish quickly, simply in a turn key way.
That said the multi-site features of eZ Publish are great!
I would build a multi-site install first for a group of similar
sites instead of many individual sites where prudent perhaps per customer.
