This was tested on an un-patched, initially released version of the game. You can do this probably with other bonus objectives that do not have a counter. In Imperial held Raxus Prime, you have a bonus objective to destroy a tractor beam tower. Since it occurs after a checkpoint, you can destroy the tractor tower over and over again (just jump off and die after destroying the tractor tower) to earn the ever valuable Force Talent (yellow) rank spheres to increase your vitality, offence and defence.