I don't believe it's devious to upgrade someone else to your own benefit, they are your upgrades to use as you wish... As long as they aren't sold or bartered, why not? There are other devious things that can be done in the program (as with just about any aspect of life), but this isn't one of them. That said, the rules should be followed, so if one uses an instrument which opens up a seat for you or a partner, then someone else wants it... There's no rightful way to prevent others from following the rules too.

The specific risk I see with the above is someone with an EF alert set for an empty or blocked seat; once the exit row opens up, someone could easily ask a GA or AC agent to swap to that seat. I wouldn't risk it.