by > GET / HTTP/1.0<CR><CR>
> a unique doc would be sent (where I could place a message saying
> we want HTTP/1.1 compatible browsers only). :-)
useful in HTTP/1.0, and many clients have been doing it for years now;
if that's all that you're trying to get, then you'll be pointlessly
turning away a large fraction of users by insisting on an HTTP/1.1
request.
Just configure your first virtual host entry to be none of the Host:
names that you want to support. That entry will then be used whenever
a call is made without a Host header, and you can send them whatever you
want to send them.
Review the Apache virtual host tutorial for details.