Federico,
Very nice find! Subtle, inspired! I had figured it as a module quirk rather than a standards-mandated behaviour. Now that you mention it, I do, of course, remember seeing that remap_503_500 parameter somewhere, but I never made the connection. It was driving me insane.
Anyway, thank you for the enlightenment. I really appreciate it. If you'll be at Kamailio World, I definitely owe you a beer.
-- Alex