As for what is a "recommendation": there obviously is a grey zone, but the examples given by the FSF shed some light on what is meant.
A sticker on the device or packaging saying "Works with Windows" would not be allowed, a note on the website saying that it is able to run Windows (amongst others) would be OK.
I don't think it would even be a problem to refer to the possibility of using non-free software like DraStic on the official website. As long as it's clear that it is up to you to install or not install it.
So if the Pandora successor gets advertized as a general-purpose computer that can do many things, including running free and non-free emulators, that is fine. If however it gets advertized as a Nintendo DS emulator, and the only way to get the advertized functionality out of the device is by installing non-free software, then the situation would be different and the FSF would not certify it.