Bypass proxy for all but a few? (reversed bypass)

I have a situation where I can only access certain websites from within my work domain. If I'm outside that domain, I can set up an ssh tunnel to a proxy at work and I can then access those websites, but I also have to have my proxy set through the ssh tunnel.

What I would like is to have my proxy settings in OS X (10.5.5) such that the proxy is only accessed FOR certain domains (e.g. use the proxy only for ' .mywork.com,128.1.*.') and bypass it for all others. This is the reverse of how the pref pane is arranged. Maybe there's some easy trick to this about which I don't know (like a minus sign: ' ,-*.mywork.com,-128.1.*.')? Anyone have ideas?

Thanks for any help,
-Gene