4 days later I've found out it's been going on for a full week, that the problem has been known and had a fix pending longer than that, and that there are no known workarounds.
... I wish the company I worked for tolerated a work ethic like that.
Still, let's try to be constructive. I'm almost certain that one of the beta patches, back before the proxies went up, let players try to use the host as proxy. If that's still possible, and if it's possible to prioritise that over using SDs proxies, does anyone know how?
At least for people capable of coordinating their games through skype, vent, teamspeak or whatever, that could be a tolerable temporary fix.