Keep in mind that whatever you would've ended up with in your secondary will now be in your keg and that you'll still need to let it sit on the co2 for a while (don't look at me for the math) once you cool it down to serving temps because of temp's effect on co2 dissolving. This seems like it might be trickier to gauge than just force carbing once you're done using it as a secondary, plus then you wouldn't have to worry about sediment from the secondary fermentation.
That said, if it works well, please post details
