As long as it gives the white health bar, then windspeaking will boost it so things like Seraph's Embrace,Steraki's Gage, Ryze passive will be boosted.
Mordekaiser W does heal so it will be boosted by the mastery. As for his passive....I do not know. Mordekaiser shield is base off a % of his health so best thing I can say is have a buddy go into a custom game with just you 2 and both of you pick Mordekaiser.
Have 1 of you with windspeaker and the other with a different keystone (but make sure the rest of your masteries and runes are the same so your HP is the same.)
Then when the game starts up see if his shield amount where is mana bar is is higher for the windspeaker user.