Success at last.
Didn't get anywhere trying to create ISOs and vmdk files with several tools.
Plop Linux didn't help either, running the boot manager just brought up a clock at the bottom right, random flashing dots instead of the star field but no menu at the top left.
Gave it one last go today, created a virtual machine with the Plop boot manager iso and got exactly the same clock, dots and lack of menu. Walked away at this stage but when I came back some minutes later I had a coherent star field and a menu. Plugged in the PathPilot install USB and made it known to the VM and the R-Drive installation screen appeared and ran to completion. There were quite a few other steps to get here, I'll attempt to repeat the process and document it.
Still haven't actually seen the main PP screen as I'm not in SIM mode and PP is looking for an interface.
Compared to VMware, VirtualBox is far more configurable.
Thanks again, Ken.