I agree that if Grevious=Maul, we'll need a "Reconstruction" scene at some point. It has to be in AotC, because you're trying to avoid loose ends from TPM, and Grevious is already running around in RotS. It would also need to be at the end, otherwise you'd expect Grevious to appear at some point in AotC. Here's my idea on how it can go down. It requires some dialogue that I don't know exists, but is fairly general. Italicized text needs to be found somewhere.

INTERIOR: CORUSCANT, SECRET LANDING PLATFORM - DAWN

The ramp lowers. COUNT DOOKU emerges and walks to where the
hooded figure of DARTH SIDIOUS stands waiting. COUNT DOOKU
bows.

COUNT DOOKU: The Force is with us, Master Sidious.
DARTH SIDIOUS: Welcome home, Lord Tyranus. I have something to show you.
COUNT DOOKU: I have good news, my Lord. War has
started.
DARTH SIDIOUS: Excellent. (smiling) Everything is going as
planned.

INTERIOR: CORUSCANT, IMPERIAL REHAB CENTER - DAY

MEDICAL DROID: We found him in the melting pit on Naboo. His body was broken, but his spirit endures.
COUNT DOOKU: But this is impossible!
DARTH SIDIOUS: Not for the Sith.
COUNT DOOKU (Realizing): Together, my Lord Sidious, we shall rule this galaxy.

The "Not for the Sith" comes from TPM, and Dooku's last line comes from The Two Towers (with slight modifications.) The final Dooku/Sidious conversation could take place offscreen, but a close up of Dooku looking suprised whilst in the shadows would help a lot.

For the reveal, could you put it at the moment Grevious escapes from the ship, near the begining? Replace "You lose, General Kenobi" (or whatever he says) with "I've waited thirteen years for this, General Kenobi." You'd still need some reaction from Obi-Wan, but the specific situation means the reaction can be more subdued, and possibly even wait until the next time someone mentions Grevious in a conversation with Obi-Wan.

I'm not sure either of these would work, but they're my thoughts on how you might be able to do Maul=Grevious.