I have come up with a plan to flip Dooku, or rather, to make Dooku flip
us. Credit goes to
Rashid for the idea.
1. We sneak into Dooku's bedroom while he's asleep. Special note: we should do it on his birthday.
2. We leave behind a cake (missing a slice, because cake is delicious), a knife for him to cut his own slices, a birthday present, and a note.
3. The note wishes him a happy birthday and a rather late "thank you" for saving our life way back when in the Omega incident. Regret the current position that politics and life has put us in, as we were fond of our lessons with him. Wish him well.
The point of all this is to both strike fear into him, the idea that we can kill him at any time we want and the only reason he still breathes is because that's what we desire. He follows Sidious now not because of loyalty, but because of fear of the man, and redirecting that fear to us (i.e. the greater, more immediate threat) will shift his "loyalty" somewhat. However, Dooku is a smart man, and though he will interpret our actions as threatening, he will interpret the content (the cake, the present, and the letter) as proof that peace with us is possible, should he just reach out to us. He doesn't know that Ventress is with us or even that she's alive, so for now he still thinks of us as that nice girl from way back who he has to consider as a liability because Sidious says so.
Carrot and stick, basically.
4. Dooku wants Sidious to die, he has no personal issues with us, and he has greater things to fear than Sidious (us, if he keeps antagonising us). He will contact us and ask us for a meeting.
5. At the meeting, he will patch things up with us, catch up on lost time, and attempt to enlist our aid with some "personal" matters. During this meeting, we should express sympathy for the loss of his apprentice. We enjoyed our training with her and though Dooku and us might've had our differences, it's always sad when you lose people to war. This is supposed to remind him of why he lost his favourite apprentice (Sidious ordered her death) without implicating us and in fact making us look better.
6. While we're doing these non-war related, secretly anti-Sidious jobs for Dooku (in exchange for cash, because it would be suspicious if we just did all this for free), he will curry favour with us and slowly draw us in so that we begin to trust him and develop loyalty to him (as a partner, not a subordinate), he will reveal Sidious to us and enlist our aid in defeating him.
7. Sidious loses control of the CIS, he becomes easier to defeat, and after his death, we deliver Dooku to Ventress on a silver platter, assuming she still cares about petty revenge by that point. If she doesn't, we kill Dooku anyway because having Sith Lords running around is bad for business and this way we can loot all his stuff.
8. "Just as planned."
This might seem like it's too elaborate and complex to some, but is it really that much compared to anything Palpatine's done?
EDIT: Sneaking into his room is also very possible. The Nightsisters did it in The Clone Wars pretty easily and Dooku only woke up when he was shot by one of their blowdarts. And guess who we're now allied with?