> Now, mlogger just silently continues not writing to history. There is no ongoing error message, there is no 
> ongoing alarm, only sign of trouble is empty history plots and history files not growing.

I would recommend to use an "internal alarm". This is not an "ODB alarm" where values are compared limits, but it is directly triggered by C code. To do 
so, call 

  if (disk_full)
     al_trigger-alarm("Disk full", "Disk full, no history will be written", "Alarm", "Disk full", AT_INTERNAL);

This will cause an alarm to show up prominently on the status page and beep every few minutes.

Stefan