I played around with "#define MCE_TKLOCK_MODE_CHANGE_REQ "req_tklock_mode_change"" as the one from the URL above is just a notification signal. Still didn't manage to get the OS to send me volume key events while the device is locked.

Perhaps there's some special handling activated by the name of the application, see the old fmtransmitter (libplayback) hack to output audio while silent mode is active.

Thanks!