Maybe I got it wrong, but you can already do something similar by setting an "Upvote Mana threshold":

In this example above - which is taken from my dashboard on hive.vote - I set a threshold of 95%, so my account follows the curation trails I chose only when my upvoting power is more than 95%, and pause when it's below that value.

I have activated a few curation trails (with a small voting weight, between 5% and 15%, for each one), so my voting power never touches 100%, but also never goes below 94.xx%.

BUT you would not earn Curation reward for it.

@cherokee4life I'm afraid that something like that would stop a lot of people from activating an option like the one you described: if you are managing your voting power so well and responsibly, almost always voting manually, maybe there's no need to decline the curation rewards you earn while on vacation!