An iptables "SOCKS" target that routes traffic via a SOCKS server, or similarly via an HTTP CONNECT proxy.
This isn't really iptables' job, though; it'd be better done using a virtual interface and conventional routing.
One cheating approach would be to build a SOCKSified SLiRP, connect it to a pty, and route to it.
Paolo Ardoino's Kernel SOCKS Bouncer does this as a patch for the Linux kernel.