Replying to myself in case others have similar problems.
I can at least get the interfaces to come up if I enable execution of /etc/rc.local (“Post Boot”) via the web interface, and add the following line to it:
At least this brings the interfaces up. Although the root of the problem is still there — because the interfaces don’t come up when they should, services such as DHCP fail to start and have to be manually restarted. At least I don’t have to be at the console to restart them, though, and adding another line or two to /etc/rc.local isn’t too bad. But it’s an ugly hack!
Still curious if anyone knows why the VLANs won’t work on the bonded interface, or where else in the boot process to try to fix this.