Select multiroom receivers remotely

[abrakadabra2k]

Hi all,
I have a working multiroom setup with 3 receivers. But rarely all three are active. Currently I do this manually via GUI checkboxes:

I would like to change this settings via http requests (like http://IP/command/?cmd=). 
I searched through the setup.txt and mpd protocol, but could find nothing that helped me.
Can someone point me in the right direction?
Alternativly to activate/deactivte a receiver I could also live with muting it, if this is easier