@calum – I think you will find that they have deliberately kept the MP4-29 blank to attract potential title sponsors. If they covered the car in the logos of sponsors they already have, then it might deter some of the fence-sitters.
Besides, sponsors only really car about having their name on the car during the season. They’re probably not too fussed about it during pre-season testing.
If they have been running higher revs so far, it would not be because they are trying to lure more sponsors by setting faster times. It would be because their testing programme has advanced to the point where they can do it. Running higher revs without sorting out the basics first would put excessive strain on the engines and would make it harder to figure out the cause of problems if something went wrong. Testing is like a blind scientific test: you change one variable at a time, and observe the effects. The idea that McLaren are running higher revs to manipulate the timing sheets and lure sponsors sounds like the kind of rubbish that fans who do not like the team would cook up.
Also, I very much doubt that they will pick up a title sponsor mid-season.