I'm not seeing a way to turn this on and off programatically,
dpms is a extension of X11 which allows us to control the display, I see no such power managment extension to X11.
There isn't one; power management is almost exclusively handled by the logind component of systemd now, and Gnome, KDE, and XFCE's power managers all interface with logind in current versions.
I'm surprised that the open desktop systemd-inhibit doesn't work with MC. That sounds like a system bug.
It seems to work here when I run systemd-inhibit as a privileged user, but when run as a normal user Gnome-power-manager just ignores it and suspends anyway. The documentation mentions that taking inhibitor locks is a privileged operation (and that privileged users can bypass the locks), but (perversely) the command line utility will register an inhibitor lock even without privileges. The unprivileged lock just doesn't actually work (at least Gnome doesn't respect it).
So I'd advise testing by placing the lock with sudo or as root and seeing if that works. To avoid running MC as root, try executing
sudo systemd-inhibit sleep 3600
Or something like that (assuming your suspend timeout is less than an hour). That worked here, and if you can establish how to get a lock, we're halfway to at least a workaround solution
I need to do some more testing, but I have a half baked script solution that avoids running MC as root. Basically it just queries MCWS once a minute to see if something is playing, and if so executes as root
systemd-inhibit sleep 70
Then the script waits sixty seconds and then tries again (or could be run with cron once a minute). It's only half written and scarcely tested (because the minimum idle suspend timeout on Gnome is 15 minutes, so testing takes forever).
The "real" solution though is to write an appropriate wake-lock using the dbus API; I'm trying to dig through it, but not getting very far. I assume that API must work as there are a number of apps (like Caffeine) that successfully use it to prevent sleep. See e.g. the freedesktop inhibitor here:
http://bazaar.launchpad.net/~caffeine-developers/caffeine/main/files