
  if I happen to need my disk for virtual machine, how would I tell the scheduler to stop?


Cron and anacron are (normally) daemon services.  You can stop and start them the way you stop and start any other service.


  cron would not be good, since it would not work when my computer is powered off


That's a pretty tough criterion to get around.  What is it you want the software to do while the computer is powered off?


  anacron might be better, but I assume that it would run all the tasks right after booting.


Apparently.  Here's an idea though: start and stop anacron with cron.


  Run only one disk heavy task at a time


Anacron has this option:


  -s
  Serializes execution of jobs.  Anacron does not start a new job before the previous one finished.


This just leaves you to implement these two:


Do not run when running on battery.
Provide some visible status. That could be through kdialog even.


The first could be accomplished by having cron check the battery status before it starts anacron.  The second could be done a wide variety of ways; you've already suggested one yourself.

Replace my recurring task scheduler with some existing product
I think the title says it all. Let's say you're mastering a game without the benefit of miniatures, maps, or any kind of physical representation of the environment. How would you keep track of details like line-of-sight, ranged attack viability, and all the other small nuances which go with creating a "believable-enough" environment in which your PCs live and thrive?

(I admit that @LoganMacRae, who is my own DM, is an inspiration for this question. He does this with panache, and in my one-off, I feel like I have some mighty big shoes to fill!)
