I think you can get in trouble with the 5 cm peak of saddle behind centre of BB rule if you use the forward position, but it's easy to try which position gets the saddle nose 5 cm behind the BB.
CSC use saddles with a cut off nose on their P3/P3C TT bikes to be able to move the saddle further foward. But remember there's also a UCI rule for the minimum and maximum lengt of the saddle (24 cm to 30 cm), so you can't cut too much off. The maximum allowed lenght used to be 27,5 cm, and many of Selle Italia's saddles are stil 27,5 cm. Fizik Arione is 30,0 cm and would not be legal under the old rule.
Here's a picture of Zabriskie (probaly the most aerodynamic TT position in the pro peloton), it looks like the saddle is in the front hole in the seatpost, but the nose of Zabriskie's saddle is also cut off to make the saddle only 24 cm. A lot of riders only sits on the nose of the saddle like this to get further forward without violating UCI's 5 cm rule.
UCI CYCLING REGULATIONS
PART 1 GENERAL ORGANISATION OF CYCLING AS A SPORT
1.3.013 The peak of the saddle shall be a minimum of 5 cm to the rear of a vertical plane passing through the bottom bracket spindle (1). This restriction shall not be applied to the bicycle ridden by a rider in a track sprint event, keirin, 500 metres or 1 kilometre time trials; however, in no circumstances shall the peak of the saddle extend in front of a vertical line passing through the bottom bracket spindle.
(1) The distances mentioned in footnote (1) to articles 1.3.013 and 1.3.016 above may be reduced where that is necessary for morphological reasons. By morphological reasons should be understood everything to do with the size and limb length of the rider.
Any rider who, for these reasons, considers that he needs to use a bicycle of lesser dimensions than those given shall inform the commissaires' panel to that effect when presenting his licence. In that case, the panel may conduct the following test. Using a plumb-line, they shall check to see whether, when pedalling, the point of the rider's knee when at its foremost position passes beyond a vertical line passing through the pedal spindle (see diagram «Measurements (2)»).
1.3.014 The saddle support shall be horizontal. The length of the saddle shall be 24 cm minimum and
30 cm maximum.
(text modified on 1.01.03).