The best source of info on Tibet is The Land of Snows, where it states (bold emphasis mine, capitalization emphasis theirs)
- Which areas of Tibet require an organized tour?
All areas of the Tibet Autonomous Region require all foreign travelers to be part of an organized tour with a Tibet travel agency, that includes travel permits, a tour guide and a private vehicle with a driver (no private vehicle needed if you are just staying in Lhasa). The Tibetan prefectures found in Qinghai, northern Sichuan, western Sichuan, southwest Gansu and northwest Yunnan are (usually) open and do not require travel permits or a tour guide.
- How can foreigners go to the TAR without an organized tour and tour guide?
You can’t. Unless you are a student at the Tibet University or have a work permit and resident visa for the TAR, you cannot go to the TAR without being on an organized tour. There are NO exceptions! Foreign travelers MUST be part of an organized tour that includes permits, a tour guide and a private vehicle with a driver.
Besides the exception for students and work permit holders in Tibet mentioned above, the only other exception they mention is:
- I am a foreigner but my spouse is a Tibetan who lives in the TAR. Do I still need to be on an organized tour to go the TAR?
The answer varies from time to time, but normally you do not need to be on an organized tour. Your spouse should be able to contact the Public Security Bureau (PSB) in his/her hometown and get you a special permit that allows you to go to your spouse’s hometown without travel permits. This normally takes several weeks or even months to arrange, but should be possible.
which still requires a permit and I guess is not really an option for most of us.
However TravelChinaGuide, (a travel agency which) I found in the past to be a surprisingly good source of info on China, states that
Note: The Hong Kong and Macau citizens who hold SAR passport, Home-Visiting Certificate or Permit to Traveling to and from Hong Kong and Macau do not need a Tibet Travel Permit.
and again here
Question: I'm a resident of Hong Kong and I have a Chinese Passport. Do I need Tibet Permit to visit the region?
Answer: No. Chinese Passport Holders do not need the permit to go/travel there. According to the rules and regulations of the China National Tourism Administration (CNTA), foreign passport holders (including the Taiwanese and Non-Chinese ID card holders need to have the travel permit for visiting. People who hold Hong Kong SAR passports and Macau SAR passports do not need the permit.
Here is a seven-year old thread on the Lonely Planet forum, which would back this up, and another travel agency saying:
Note: Hong Kong and Macau Citizens can visit Tibet without Tibet permit.
There also seems to be an exception for "Taiwan visitors who hold a valid Chinese ID card" but that opens a discussion I do not wish to get into.