I have a very simple solution for that: you get to choose: you are either the provider of commercial software and accept liability for your product or you have to open source it.
I don't know what you're getting at, that's clearly spelled out already. But in case you didn't get it: that's without liability for the manufacturer because you have the choice to bypass them. Presumably such a situation wouldn't occur very often because 'proprietary hardware' with 'open source software' wouldn't be proprietary for very long. The software would tell you all you need to know about how it works.
Yes, if you have a point to make you should spell it out. Google manufactures phones, has an open source OS and has a bunch of proprietary stuff that they do not release, see:
"However, most devices run on the proprietary Android version developed by Google, which ships with additional proprietary closed-source software pre-installed, most notably Google Mobile Services (GMS) which includes core apps such as Google Chrome, the digital distribution platform Google Play, and the associated Google Play Services development platform. Firebase Cloud Messaging is used for push notifications. "
