The reality is that if you are using a computer OR tables to control your dive/ascent, you will be in complete breach of the built-in algorithm by suddenly making a radical deviation from the plan. I think there's a very good chance you'll end up on oxygen and then in a chamber, but that's still preferable to the alternative in the cited scenario.
I watched a diver do something very similar some time ago, having run pretty well up to the limits on a recreational dive he dropped back to maybe 50ft from his final stop ("safety stop") to photograph something he'd just spotted on the bottom. That was a costly mistake. He returned home about a week later than he had planned, and some $40k poorer. He hadn't thought he needed dive insurance - he was wrong.
---------- Post added July 12th, 2013 at 09:00 PM ----------
Your comment was far from a general one. You implied that using a computer was a cop-out and a real "tech" diver wouldn't need/use one. That is supreme arrogance. Whatever a "tech" diver is, (s)he should be equally comfortable in either case, but interested in using a computer because of the greater flexibility it permits. I was using myself as an example of many experienced divers out there - I have neither the need nor the desire to justify myself on a message board.
What you are describing with the photographer does not relate to the Dumpster diver incident....
In your story, the recreational profile which we can assume had been "maxxed", got interupted at the final stop--and the diver in question was potentially at a borderline condition of bubbling at this moment--and needing to avoid a cascade effect.
If this photographer is going to shoot something , this usually means he is going to spend at least SEVERAL MINUTES in setting the shot up, arranging strobes, trying for multiple pictures and angles. While he was doing this final shooting, this photographer actually extended his bottom time --making the dive that many minutes longer, and I think it is safe to say that he turned it into a decompression dive--made worse by the bubble growth he had from the bubbles that had already been created at his initial safety stop. From your account if the incident, it does not sound like your diver treated the dive as a real deco dive, with a real decompression stop at 30 feet or 20 feet, after the final shooting.....
In the DD example, he would have needed a single minute or even less to have the big ship pass--and then he would go back to a REAL DECO stop.....which ever one he had been in before....if he had pure Oxygen( which would have been smart on a deep dive scenario as DD had suggested), then the bubble dynamics would be handled by doing the full stop duration at 20 feet on O2. If no O2 had been taken by DD( or his stand-in), then one stop depth higher might well be the solution---ten minutes at 30 feet, and then back to the 20 foot stop. Also keep in mind that the deco time was for a relatively short duration 160 or 200 foot deep dive...and that at 50 or even 70 feet deep, this is still technically a decompression stop depth, and the gas dynamics important to consider at this time-- are the issues with dissolved gas from 200 feet deep, not from a minute at 70 feet.
This is not about complex math...it is really more about common sense.