You can add a timer to save the join count every few seconds. (It'll overwrite the jcount.dat with the latest info) This way, you don't have to worry about losing it:

on *:START: { 
  hmake jcount 100 
  if ($isfile(jcount.dat)) hload jcount jcount.dat 
  .timerjcount 0 15 hsave -o jcount jcount.dat
}

I put it for saving every 15 seconds, but you can put any second of your choice that you see fit. The shorter the second, the frequency of saving it becomes.