Run custom task automatically before/after standard task

Extending an existing task is documented the SBT documentation for Tasks (look at the section Modifying an Existing Task).

Something like this:

compile in Compile <<= (compile in Compile) map { _ => 
  // what you want to happen after compile goes here 
}

Actually, there is another way – define your task to depend on compile

prepareAppTask := (whatever you want to do) dependsOn compile

and then modify packageBin to depend on that:

packageBin <<= packageBin dependsOn prepareAppTask

(all of the above non-tested, but the general thrust should work, I hope).