So my beloved HP Z620 Workstation Monster has just suffered a BIOS update failure.
It seems that this is a common issue with running the update from within Windows 10.
Following various post including this one seem to have not worked: https://h30434.www3.hp.com/t5/Business-PCs-Workstations-and-Point-of-Sale-Systems/Crisis-Recovery-Jumper-Z620-Z420-Z820-Revealed/td-p/6658619?lightbox-message-images-6658619=179776i02BEED432C66A811
$110 later, new board on the way. Seems that whatever happened has super broken it to the point even the recovery ROM does not function.
Removing the RAM triggers an error status with beeps and red lights meaning it doesn’t appear to be completely dead.