seabios doesn't read the log, instead it'll append its own messages.
Which I expect will continue to work fine, except when it hits the end
of the log buffer, where seabios will simply stop logging instead of
rolling over.

Right, sorry, I confused something while writing the explanation there. Nevertheless, the SeaBIOS code will require a small adjustment (same as the libpayload driver in the patch I linked) to continue appending correctly after the log has rolled over, and I'll provide that patch.

Will the ability to change the 'Room allocated for console output in CBMEM' size still be available (and functional) for those need to view the full console output due to the addition of debugging/tracing lines?

Yes, nothing changes about where the console buffer space is allocated. You can continue to increase the size through Kconfig. I'm just changing how it gets used.