What would satisfy me, and "Accept as Solution" is if a HP Quality Engineer or HP employee would post that if I buy an HP branded Quadro card to install in my covered Z820, that HP would support that card under my HP Care Pack warranty.
Call HP Sales and ask the question there. Telephonenumbers can be found at the HP site for the contry you live in. 
I do really understand your concern since i care very much about the HP Care Pack warranty as well. What i do know about it that HP are picky about selling non-certified items to a workstation. I had issues buying cooling fans to a Z800 that were certified for a Z820. (Z820 were much quiter)
I would have called sales to ask them.