It's a tricky one, certainly - and I do think CGs are a poor fit for Powerplay in general because all the mechanical stuff Powerplay already does better.
- the CG itself has no direct Powerplay effect, whoever wins
- it obviously has a very large indirect Powerplay effect by bringing unusually many players to a particular system to do stuff which also (if they're pledged) might score Control Points for one side or another
- most CGs if they take place in a Powerplay system, because of how broken the Undermining activity selection is, are just free reinforcement for the owning Power despite being on paper outnumbered ~11:1 (and that includes, of course, CGs with nothing narratively to do with Powerplay at all)
- these CGs do at least use one of the few symmetric Powerplay activities so the Undermining levels are keeping rough pace with the Reinforcement taken across all six systems rather than being "hey, free Stronghold"
As far as total Powerplay balance goes...
- ultimately it's irrelevant to the outcome, it's six systems of Duval's at stake, none of particular importance, two of which on current rates they'll lose by the end of the two week period, during which time Duval will probably gain fifty more elsewhere, and the two they'll probably lose they have a very good chance of just taking straight back (if they want them) once the CG is over.
- while no Power gets attacked very much in normal circumstances, one of the reasons for Duval heading for the top of the leaderboard is that they usually get attacked less than most. So the CG levels the odds somewhat on that.
- overall Powerplay balance (even just in terms of how easy different Powers find it to Reinforce/Acquire their own systems peacefully) is a sufficient mess as things stand that a CG loosely targeting one of the beneficiaries of that imbalance is a very crude hack but arguably better than doing nothing
So I think as a long-term goal, this sort of thing shouldn't be needed because it should arise naturally out of people playing Powerplay. But it hasn't been and so many changes would need to be made to make that happen, so as a short-term patch over that it's fine.
(Whether Frontier actually understands why there's no real inter-Power hostility going on in Powerplay is hard to tell. Most of their rebalancing so far has been largely irrelevant to it)