On inferno difficulty, uniques and champions will automatically heal back to full after a certain amount of time if you aren't actively fighting them (10 seconds or so). If I were to drop a sentry before I die, can this keep the boss engaged enough to prevent them from healing up in time for me to get back into the fight?
No the Sentry depsawn when you die.
It is on the other hand possible to extend the reset timer with a follower the restart timer will not start untill the follower is knocked down.
At least that is what I have been experiencing.