coreboot

The behaviour should be configurable. There are 2 bits reserved to configure this behaviour.
See bit 5-6 (count from 0) in 0x3 of the EC memory.
You can change those bits using ectool under coreboot.git/util/ectool.

./ectool -d
dumps the whole memory (a special buffer on the EC).

Read out the byte on 0x3 and play a little bit with bit 5-6
./ectools -w 0x3 -z 0xXX

So do we implement an nvram parameter for this (so it is static and needs a reset/s3 cycle for it to be applied) or an ACPI method to change it during runtime with the acpi_call module or both?

The thinkpad_acpi kernel module knows about the "hardware mute" and should be able to handle it, as it's not coreboot specific. I'll do some tests on T500.