I don't have any actual experience with this but I doubt it will have much affect at all. I've read that cannabis really only looks at the length of night, not the length of day. Which is why a 6d/12n cycle will flower plants and get you to a quicker harvest (at the cost of yield due to shorter day length).
Sorry, wish I could supply better first-hand experience or STFU; I thought it might be useful food for thought nevertheless.
There's a technique called the 'buddus interruptus', let me give you the skinny from "The SCROG Bible":
"Also known as ``buddus interruptus'', the procedure is to switch the lights to 24/0 for a day or two about at the end of the fifth week of flowering, when the buds seem to stall out. It should only be done once during the crop, and for no more than two days. The best technique is to switch for one day, wait a couple of days to observe the effect, and then give it one more day if the buds haven't responded. This is not the same as ``double budding'', as the plant is never actually kicked into vegetative growth.
If the plants react well, you will see tufts of additional flowering parts standing out from the sides of the buds like little towers. Done early enough, these extra parts should finish in time, and they will add extra weight to the crop. This technique can be particularly valuable in scrog, given the large number of smaller buds.
Not all type of genetics react well to this though, it's trial and error.