Hello @ncw,
I take it the problem is that rclone will choose IPv6 connectivity by default, but that doesn’t work 100% reliably? Is that it?
IPv6 'works' 100% reliably on my current setup: as my ISP doesn't offer it, it fails every damn time 
What isn't "reliable" is whether rclone uses the IPv6 address or not; I'm not sure whether Google's DNS servers won't always return IPv6 addresses or not, or if it's rclone not picking them by default every time, but whatever it is, it makes the end result very unreliable: even after I remove IPv6 completely from the machine (eg, echo 1 > /proc/sys/net/ipv6/conf/<interface-name>/disable_ipv6) , rclone sometimes starts trying to connect to Google servers' IPv6 address... and in the middle of a large sync/copy/cryptcheck, it screws the entire process... 
Nice reminder about /etc/gai.conf; I was not aware that anything used it these days here on my machines it either doesn't exist (FreeBSD11), is a zero-byte file (EL6) or is full of comments (Devuan)... but I will have a look and create it on my rclone machines and so some experimenting.
Otherwise your BIND solution seems like like a reasonable one if you really don’t want IPv6 any more! As there are very very few IPv6 only services (can’t think of any except for test things) I don’t see a downside.
Don't get me wrong, I have nothing against IPv6. But the fact is that the local ISP here don't have it (yet!) so it's no use keeping it configured and dealing with the resulting issues... when the ISP gets a clue and starts offering it, or better yet, I get a less-sucking ISP, I will certainly reconfigure my BIND named server to let IPv6 records through again...
Thanks for helping me again, Nick. I will keep this topic posted.
Cheers, -- Durval.