There is no way to get BOINC to do it automatically. Doing what you are suggesting does not always work as one might desire. Sometimes the task you have promoted will get sidelined after running for about an hour, then get sidelined for several hours.
As you say suspending groups of tasks is not without risk (phone rings, baby and dog decide to have a puking contest, cat pulls down curtains and domestic boss calls to say dinner is ready.....)

Since my ears are burning (though I can't be sure you were referring to me) - why would you want to do this, anyway?

When i micromanage things by suspending some tasks to allow others to run, it is nearly always to allow testing work units to run which I keep a much closer eye on anyway in order to give maximum information back to the project so I keep the suspended tasks suspended till the testing branch task is completed.

I can't remember which are shortest but certainly exclude Africa Rainfall Project as these are by far the longest, taking about 37 hours on my also old and slow desktop.

Edit: Have you tried asking on the WCG forums?

You were answering to a spammer