Worked this out... The supplied SOE on these devices includes a HP Encryption Software (which is actually a rebranded McAfee EEPC). Uninstalling that allowed McAfee EEPC to install without a problem.
Pre-Boot on the HP Revolve seems to be acting a bit strange though... Seems to break the touchscreen....
We are working with the OEM to resolve a digitizer issue (touch / finger based digitizer).
Good to know. Was suprised by the problem given that the HP bundled software was based on EEPC.
It's quite annoying having tested 4 different devices from different brands (Lenovo Helix, Dell XPS12, HP Revolve & Samsung Slate7) and all their quirks with EEPC.