I've just tried applying the modified firmware image to my bridged VMG1312-B10A. I was already running V1.00(AAJZ.14)C0 which this modified firmware is based off so figured it would be a relatively safe upgrade. And indeed it was, modem rebooted and came back with the existing config intact. Updated the settings on my firewall to request a 1500 byte MTU on the PPPoE connection and bingo, it's working!
The modem's ptm0 interface reports that it is at 1508 bytes;
ptm0 Link encap:Ethernet HWaddr B8:EC:A3:75:AA:30
inet6 addr: fe80::baec:a3ff:fe75:aa30/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1508 Metric:1
RX packets:84797 errors:0 dropped:0 overruns:0 frame:0
TX packets:62019 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:73633078 (70.2 MiB) TX bytes:9125407 (8.7 MiB)
Pings work up to a payload of 1472 bytes, confirming that a 1500 MTU is working (1472 byte payload + 8 byte ICMP header + 20 byte IP header = 1500 bytes);
>ping kingj.net -f -l 1472
Pinging kingj.net [195.154.46.25] with 1472 bytes of data:
Reply from 195.154.46.25: bytes=1472 time=21ms TTL=53
Reply from 195.154.46.25: bytes=1472 time=21ms TTL=53
Above 1473 bytes payload, fragmentation is required as expected. Confirming at least that there's no secret fragmentation going on on the modem side!
ping kingj.net -f -l 1473
Pinging kingj.net [195.154.46.25] with 1473 bytes of data:
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Very pleased that this works! The BCM63168 and the VMG1312-B10A's filtering works great on my line letting me sync at a full 80/20, but it was always a shame to have no baby jumbo frame support. So very glad that's no longer a problem.