I used DISM to update the image with .net (You need to extract SXS folder from the win8 .iso)
dism.exe /mount-wim /wimfile:C:\DISM\Win8x86_golden.wim /index:2 /mountdir:C:\DISM\mount
dism.exe /Image:c:\DISM\mount /enable-feature /featurename:netFX3 /All /Source:C:\Temp\sxs
dism.exe /Unmount-Image /MountDir:c:\DISM\mount /Commit
I have imaged about 30 of these devices, 30 more to go. Some seem to just randomly fail at the BIOS update stage, annoying. I see HP have a new Driver and Firmware Update so may give that a go