You could create an execute file action, and execute the dism command with the /forceunsigned option that runs after HII.
We too are seeing this issue with the Dell Optiplex 740 SATA Controller driver.
I am trying to do what Eric suggested at the moment. I will report the results we see.
It would be nice to have the option in the HII Driver Management tool to add switches and things to specific drivers that are assigned to specific devices. That would eliminate having to create an execute file action that runs on every computer, especially when they are not needed. We use the same templates for all computers so the HP computers will run this command unnecessarily.
I was able to do what Eric suggested, and got the driver to install that way and things worked fine after adding the Execute file action.
I got lucky though and was able to find a driver that HII was able to use without throwing the Error about it being unsigned. Thanks for the tip Eric.