Thank you for your post.
Is your GPON SFP module plugged directly into the ER8411?The ER8411’s 10 G SFP+ WAN port only supports negotiation at 10 G, so the link will not come up.

I am aware that the link will not come up. It is plugged directly into one of the SFP+ ports on the ER8411 and I've manually set the ER8411 to negotiate at 1G, which allows the link to come up. As I wrote and linked in the thread above, the underlying software (Linux) of the 8411 will happily negotiate at 2.5G as shown by the logs, but nothing is ever visible from the UI. It seems like this is a limitation of the management software (controller/gateway), rather than the physical port or the operating system, which is why I asked whether support was planned to be added to the controller/gateway firmware to support this. 

The MC420L is currently not sold in Canada, and there is no information on whether it will be available there in the future.
As mentioned in the thread, if your modem supports an Ethernet connection, you may consider using the SM5310-T instead.

Noted WRT the MC420L being sold. It seemed like v2 of this product was meant to solve my exact situation, so without Omada itself supporting the 2.5G link speed, or the hardware being available I'm stuck without being able to get full speed from my connection.

No, there is no modem. It's an SFP GPON module. The SM5310-T does me no good -- I would need a media converter that accepts 2.5G SFP and outputs 10G ethernet, which is what the MC420L v2 would do for me.