To reduce the amount of work required to maintain the list I'd propose to remove the emailing step (or at least reduce it to a one way email without the need to parse replies) from the process and use ContactInfo as a way to collect the intention of relay operators to opt-in/opt-out of the fallback directory mirrors list.
This comes at the price of increasing the descriptor size, but (temporary) 5 chars might be ok? You can probably reduce it to 3 chars if 5 is to much.
This could be something like:
Add " fd:0" at the end of your contactInfo for opt-out and " fd:1" for opt-in.
or f0 and f1
Once you defined a format you could simply email that info to tor-relays and known fallbackdirs and watch how well it is received by looking at contactInfo data.
This way you could probably update the list with every release without requiring a lot of manual work.