Hello Matt,
I'll try to help you... Please, do understand that I did not get well what really you are trying to do. Let us do one step at the time.
This step: 2) video output works properly for SeaBIOS and grub/syslinux, but output is disabled once the OS / kernel driver loads.
_______
What I am getting from this email is the following (correct me if I am wrong): BYT-FSP -> Coreboot -> (payload) SeaBIOS -> grub (2.0???) -> Linux kernel 3/4.x.y (?).
Now. If you use as payload SeaBIOS, my best understanding is that you'll use CSM (Compatibility Support Mode). So, in other words, you'll use (if you will?) in Coreboot vBIOS (not GOP driver). Now, furthermore, you MUST use vBIOS, since you are using SeaBIOS. And Linux will use vBIOS (not GOP driver), since you'll pass INT 0x15 mechanisms for Linux GFX (using mandatory vBIOS passed from Coreboot), enforced from SeaBIOS - CSM?!
The question here is the following: why, for the change, you do not use as payload TianoCore? This one is UEFI compatible, and very well suits UEFI compliant Linux? In other words, you will use Linux as UEFI compliant/compatible OS. Compliant to Tiano Core, which brings to you UEFI features (initialized by default with Linux). Simply and plain... And see what will happen?
Final line: I suspect, you did not built-in in Coreboot vBIOS package and vBIOS init (just serial output), which is, using SeaBIOS payload (CSM mechanism), I guess, mandatory (for Linux to overtake/inherit legacy, to work with GFX).
Thank you,
Zoran