Just reporting back to let you know that WiiM Support has replied to my ticket to state that the "Pro Plus does not support DC protection".
Now, the next thing to wonder is whether, given that surely there is at least one person out there in the world with a Hypex NCore amplifier running out of a WiiM Pro Plus, the DC output of the WiiM is low enough so as not to be a concern, assuming there is no DC offset in whatever track is playing.