So, yes, making a GPL request will work for the very few components still under GPL, if a vendor releases a binary patch. But for most things outside of the kernel, patch diffing comes back into play, just like on every closed-source OS.
I would understand in a modular system like an operating system: one can argue that the kernel is a single component.
But if you're buying an appliance, the OS is effectively one single unit: all linked together.
Why does a binary executable and a binary image seem to operate differently in this space - both are inscrutable?
In practice, GPLv2 would not be viral in the way you describe unless you can show that all of Android is a derived work of Linux (not true). GPLv3 would require users be avle to replace components under said license which has an impact on how such an appliance need to work (though the GPLv2 does also have somewhat related text about "the scripts which control installation") but wouldn't expand the scope of code under the license, just the terms.