dealing with exhausted list of work items

[Christian Meesters]

Dear All, dear Ole,

is it possible to let parallel trigger a command once the number of 
work-items is exhausted?

Background: When running an ill-balanced 3rd party software on a cluster 
parallel is of great help to achieve optimal load balancing. With 
relatively short run times of the assumed 3rd party application there is 
no further issue. However, when the run time is substantial for some, 
but not all instances of that application it can and will happen that 
those instances occupy a node whilst the remaining nodes are all idle. 
Many schedulers allow to resize a job under such circumstances.

Screening the docs did not point me to an option to inject a command 
(e.g. HERE-doc or other script) to be carried out, when there aren't 
further work-items (tasks to be launched) - but still running tasks. 
That would be a great advantage, however, as it would save tremendously 
once fair share in any cluster environment.

Does anyone happen to have an idea/solution for this challenge?

Best regards,

Christian Meesters