Improving on this "catch all"?

Sorry about that. I didn't test the syntax first. You can't have comments in the switch cases:

http://www.tcl.tk/man/tcl8.4/TclCmd/switch.htm

Beware of how you place comments in switch commands. Comments should only be placed inside the execution body of one of the patterns, and not intermingled with the patterns.

So it would need to be like this:

when HTTP_REQUEST {
    Parse the HTTP host header value
   switch [string tolower [HTTP::host]] {
      site1.domain.com {
          Host was site1, set the pool to site1 pool
         pool pool_site1.domain.com
      }
...

Aaron