I think you may be right to some extend with what you state for an IF lens (although Chris's example above shows some image area expansion even with the Contax 35mm f3.5 lens which is an IF lens but less so than the Canon he also posts)... But on a lens without IF, wouldn't "focus breathing" increase the image circle? (without affecting image quality)... it should, shouldn't it? ...I mean the frame on the sensor will look the same with no movements applied (as of constant focal length), but the total image circle projected should be one of wider focal length (due to focus breathing) ...shouldn't it?
EDIT: At least Chris's results on the Canon 24mm above, (which is not IF) show some considerable image circle expansion when the lens is focused to its minimum... Now as far as IQ is concerned, one can't be certain that it will be less... It will be altered to some degree alright, but it has as much chance to be better as much as to be worst (depending on how near to the "optimum" image circle one is)... but in any case, it can't be much different as to seriously be considered to be a problem.
On a lens without IF there is 100% no focus breathing, if you just move the lens back and forth you don't change the angle of the lens.
If you have a lens that's affected by focus breathing, the image circle may also change - for better or worse. Without testing the lens one can't tell you that for sure, it depends on which element moves where.
The image quality will definitely be worse if you use an IF lens at close focus and then back-focus to infinity with the camera. The internal focusing is (or rather should) be set to achieve the best performance at the adjusted focus point, at close focus position certain elements will most likely be moved back and/or forth to improve the quality at a larger magnification - whether that affects field curvature, central or corner sharpness or any other aspect of the image.
As I already mentioned there's another possible problem with the technique you want to use: depending on the lens design, internal elements or the length of the lens barrel could possibly reduce the image circle more than expected. Normally that wouldn't be an issue as they're not made to be used in tech cameras with movements - but in your case you could get a smaller image circle than you expect.
Not trying to get you down, I just want you to be prepared for that.
Now after reading your last comment I think I understand what you're trying to do but either I'm completely wrong or you've mixed up some things:
If you set the lens to its closest possible focus setting, the point where infinity focus lies is CLOSER to the lens. Not further back.
If you put the camera on an adapter (which increases the distance) and your cameras sensor is another 18mm further back - then it's like introducing a bunch of extension rings. You won't get closer to infinity, you'll do the opposite.