I just tried the latest Google Now with Madsonic in terms of playing music. The first time I did it, I said:
"Ok Google, play Monster by Skillet". Google Now brought up a card asking me to buy the song from the play store as the default action. When I clicked on the play store icon in the lower left corner of the card, it presented me with all the music players I had installed, including Madsonic, which I selected. The song then started playing in Madsonic, and subsequent commands played directly in Madsonic without intervention.
However, trying to play an entire album or artist didn't work. After watching on screen what was happening, it looks like Google Now simply shunts a search into Madsonic, which then plays the first item returned in the search. The search results are usually displayed on the screen and the playlist can easily be amended from there. I don't exactly know what search is being passed into Madsonic. I don't know if there is even a way to differentiate an artist request vs. album request.
Anyway, it's better than nothing and if you only want to listen to one song at at time, it actually works.