As our corporate network provides an HTTP CONNECT proxy for people who need to connect to the outside world, and as every Linux application needs to be set up specially to use it, I was thinking about installing a service to transparently forward TCP connections through the proxy. I could just hack Slirp to connect to a tun network device on one end and to the proxy on the other, but I was wondering whether something on those lines already exists before I start doing it (probably badly) myself.
Edit: tun2socks seems to do what I want for SOCKS proxies.
Edit: redsocks uses the host firewall instead of a tun device but looks like a candidate.
Edit: slirpvde (resp slirpvde6 when it settles down) with a tap-connected vde_switch in combination with proxychains might be what I settle for. I will add it as an answer if I do.