Are there dual-citizenship holders within the Armed Forces of rank O-7 (General Officer) or higher? What percent, if any? Which nations, if any?
Since 2011, no.
In 2011, the US Army issued a personnel regulation in which the Army announced that dual citizens of the United States and another country are not permitted to enlist in the Army, Army Reserve, or Army National Guard into jobs that require a security clearance. Not every position requires security clearance, a cook may not need it, but you ask about the officers that must have it.
The United States does not recognize dual citizenship. As far as the US government, including the military, is concerned you are a US citizen, a US resident (green card holder), or a foreign national.
There is a program for US residents to fast track their citizenship by joining the military.
while I am not aware of any recent cases, there were American citizens that join foreign militaries in both WWI and WWII, usually French, British, or in the case of the fly tigers in WWII Chinese.
The closest you will come to dual citizenship is an acknowledgement that other countries may also consider you to be a citizen.
While she was not in the military, I was, my wife became a US resident and later a US citizen, but she did not turn in her foreign passport, so that country still considered her a citizen. She never use that passport and it expired long ago, but she was not required by the USA or by the country that issued it to turn it in.