I guess the flashing itself should not be that much of a problem (see https://gist.github.com/elvisizer/2dfa119e959ae6f35389d4d56054f067 for further description)
The main point AFAIK is to inject a newer NVME driver into the (pre 2015) SCAP EFIPayload file that needs to be flashed.
If I understand correctly, @y-h-wong raw-edited the SCAP file (with hex editor e.g.) and copied the NVME driver from a newer Bootrom into the update for your pre 2015 MBA/MBP. The SCAP files can be extracted from the latest Mojave InstallESD (Packages/FirmwareUpdate.pkg/Scripts/Tools/EFIPayloads).
Ok I just learnt that you can replace the NVME part using UEFI-Tool. This means, a new SCAP file can be built like that.
I'm just not yet confident enough on this to flash it using the mentioned commands, as I don't have any flashing hardware available right now and don't want to kill my system in case something goes wrong.
Ok, I now understand the UEFITool will only provide cap files, not scap files - meaning the file is not signed. I guess that's the reason why simple injection of the NVME section of the scap does not work like this.