I don't have the PMBus spec (I'm not an Adopter), but I wonder(?) if sending ByteCount=0 in the Write transfer is legal. If so, you could send that as the second byte (fulfills second TXIFG=1) then watch for the third TXIFG=1. If you get an ACK on the first byte, it will send the 0x00 byte but that would be benign.
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