Well, it's not perfect but... Here's what I did and it's at least serviceable.
I set up a sub-domain of preview.maindomain.com. Then I created vhost.conf file for this sub-domain and stuck the following in it:
[php]
AliasMatch ^/([^/]+)(/(.*))? /home/httpd/vhosts/$1/httpdocs/$3
[/php]
Restart Apache and then you can kinda-sorta preview the new domain by pointing your browser to
http://preview.maindomain.com/otherdomain.com
The problems with this quickie method? Scripts won't work for the most part (permission issues). Any relative links will not appear, this means for images, external js files, php includes, etc. Absolute references are a must.
Far from perfect, but for plain old html sites it's at least workable.
Squire