There are a few issues with this approach, though the theory is fine. It's no different in a way than bottle conditioning using priming sugar. I've done it a few times.
One issue is timing it correctly. Some people are confident enough in their process that they simply transfer with 5 points of gravity remaining, and that will, assuming about a 64 degree ferm temp, get you about 2 1/2 volumes of carbonation. Crash the beer when fermentation is done and voila! Carbed beer!
But how do you tell how many points remain? You can take samples, of course, and if you're careful and since the yeast is still working, O2 exposure should be limited. Some people use a TILT hydrometer to monitor this. I have one, and have had success with it, though it's not perfect.
Another issue is that you need to be there when 5 points remain, and transfer. What happens if it's at 5 points at 7:30am and you need to be to work by 8:00? There's a relatively small window during which you can transfer, and if you miss it, you miss it.
This can be managed by using a spunding valve and transferring with more than 5 points remaining, so you get the carbonation you want.
A third issue is that when you crash the beer, you'll have a fair amount of yeast/trub at the bottom of your keg. You can manage this if you want with a floating dip tube so you draw beer from the top of the beer, not the bottom. Others cut the liquid dip tube a bit shorter so it's not drawing quite so low.
As for me, one reason I went to kegging was so I didn't have crap at the bottom of a bottle of beer. This method duplicates that, which for me is moving backwards.
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@Iseneye describes the process I often do: I run the CO2 produced in the fermenter through the keg, purging it. CO2 from a bottle isn't pure (unless you pay more to get pure). Typically 99.5 percent is CO2, the rest is air. Air is 21 percent O2, so you have .1 percent O2 in your bottled CO2. Purging the keg with fermenter CO2 is using pure CO2, so you're eliminating that small amount of oxygen. I go further; I fill the keg with Star-San, then push that to the next clean keg using bottled CO2. That gives me a nearly perfect atmosphere in the keg. Feed the fermenter CO2 into the purged keg, and it's as perfect as you can get.
Since 5 points gets you about 2.5 volumes of carbonation, if you have a 50-point drop (say from 1.060 to 1.010), that's 25 volumes of CO2. Purging your keg 25 times will get you as close to perfect as you can get.
I don't like using bottled CO2 to purge kegs, there's a cost, but it's not that great a cost. So for me, that's the process.
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Depending on your fermenter, if you're racking and allowing air to come into the fermenter as it drains/racks, you're exposing the top of the beer to oxygen. What you can do is feed the displaced CO2 from the keg back into the top of the fermenter so air isn't entering. The pic shows me feeding back to an airlock--I cut off the top so it would accept the tubing--but I've since cut a small section of rigid tubing from a bottle filler and I stick that through a stopper when I do this.
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Given that I don't really care for having crap in the bottom of my keg, but I do believe in the value of limiting O2 exposure as much as possible, my current approach is to seal up my stainless Spike conical with about 7-10 points remaining, and letting it finish under pressure. It ends up giving me about 13 psi inside the fermenter when finished, and the PRV on the fermenter controls that. That's at about 64 degrees, and results in about 1.5 (give or take) volumes of carbonation. I then pressure-transfer to a purged keg, so that for all intents and purposes I have no oxygen in my beer at that point.
It's not fully carbed, so I'll put it on the gas to finish. This adds a small amount of O2 to that beer that comes along w/ the bottled CO2, but to this point I've not been able to discern that in the beer. Some people, I think, are very sensitive to oxidation flavors in beer, but I can't detect such a small level myself.
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