Ship list of fallback directory mirrors on long-term fixed IPv6 addresses

legacy/trac#6027 (moved) enables clients which cannot talk to IPv4 relays to use fallback directory mirrors with IPv6 addresses and ORPorts. It's going to get merged into 0.2.5.x.

Assuming this code is in tor, how would we ship an up-to-date list of long-term fixed directory mirror IPv6 addresses with bundles? I think I can provide a simple script or set up a service generating this list. But what's the easiest way to include the step "Grab new IPv6 addresses file and put it into all bundles" in the process of making bundles? There's no code written yet, so please say what's most convenient for bundle makers.