2 weeks ago we got a 1 week tour into Tibet.
We contacted several companies before hand, and they all said that we could not do it as there were only 2 of us, one British and one Swedish.
We had been trying since July, with no luck.
We tried another one a few weeks before turning up in Nepal, www.itournepal.com and they tried really hard to get it sorted, and it paid off, as we got the visa and permits in the end from these guys.
The trip was great, fly into Lhasa and then drive back to Kathmandu through Tibet. We had a Tibetian guide and driver all the way, and it was great.
While I realise the visa problem is very volatile and can change easily these guys were great with everything and I would highly recommend contacting them to see if they can help.