Yes, this can be done, with slight modifications to your plan. If you place the proxy servers inside a separate network segment, you can then intercept the outbound HTML traffic and redirect it, either load balanced or in a failover configuration to your proxy servers. We did exactly this configuration for a client who was pushing sustained 100mb through that system and it works flawlessly in 1.2.3. If I remember correctly, there were some minor modifications that needed to be done to pfSense in order to support load balancing from a CARP IP and that work was done by the BSDPerimeter guys under contract (can't recommend their work highly enough btw). I suspect all this can be done off the shelf inside pfSense 2.0, but I've never set it up. As always, the limitation of this setup is that only HTTP, HTTPS will not proxy transparently.