Most/all of this has been said before, but reading the OP again these specific points seem to pretty well explain why the specific rule change suggested in the OP wouldn't be a great idea.
Mick wrote on 08/24/14 at 02:15:31:MK8PP, where at the present time the public cannot hack time trials.
would only apply for the MK8PP. The policy may also change were dangerous hacks publicly released.
There will be a time when the public can use hacks, and a decent chance that time will be preceded by a time when one or more people looking to cheat can, without the public knowing.
The vast majority of times are without proof. Cheating is easy without hacks, even if harder to avoid detection in the long run.
Allowing players to join only temporarily is just silly. Nobody much wants that, and it doesn't discourage cheating again. If past cheaters were allowed after hacks were publicly available, why would this be only for the MK8PP? (and the present situation of the public not being able to hack becomes irrelevant)