Rugby Union has got itself into a horrible, horrible mess with the TMO. I would suggest that it's time to stop digging a deeper hole.
In theory, there is no reason for penalties not to be looked at by the TMO. If you can review the lead-up to a try with the TMO, why not penalties? Both can lead to points, they're not necessarily the scoring act, so if one can be reviewed, the other should be as well. If acts of "foul play" can be reviewed, including deliberate knock-ons, there is no logical reason to prevent standard penalties being looked at either.
However, the problem with the above is that in the end you can conclude that everything can be looked at by the TMO. It's neither practical nor in the interests of the game to attempt that. The logical issues with the current TMO protocols show how badly that will end. We get more decisions right is usually the argument used by those in favour of increased TMO usage. Yes, you do but it's never enough. Perfection is what is being misguidedly sought here. It's time to return to establishing clear factual decisions (grounding, successful kicks) with the TMO and allow an intervention if an act of foul play is worthy of a red card. Unfortunately, you're simply going to have live with the rest if you still want a game.
If you allow to penalties to be reviewed, one day someone will ask "what about the pass in the previous phase that was forward"? What about the penalty that was missed? Does the referee stop the game if he has a possible penalty for a ruck infringement? You can only imagine if the referee wants to look at a scrum to check that he hasn't missed something. It's a never-ending process. A process in which I think it's time to stop digging.