Just an update in case anybody was watching this (the original) post - I've finally published the guide... I hope that you find it helpful - please feel free to ask me any questions (as I've not had time to polish it quite as much as I'd like)
Details are on the forum post here:
A direct link to the guide is here:
Recording Live Gameplay in RetroPie’s RetroArch Emulators Natively on the Raspberry Pi