Silly ass question

OK, way back when I was young (we are talking pre-bud days), we used to get chunks of dry ice, put them in a can of water and let the fog that came out flow down over our plants. Our thinking at the time was that plants "Breath" Carbon Dioxide, the fog from the dry ice was pure Carbon Dioxide, so it would boost the plant. We didnt do it often, but it seemed like the plants would really get a boost in growth after we did it.

Does this actually help the plants out?

In theory, yes it should work. Dry ice is just frozen carbon dioxide, so the fog SHOULD be pumping the room full of co2. Never seen it done or tried it myself though. Proceed with caution.

It would need to be continously through flower not just a day

This has been talked about on here for years and yes it is feeding co2 to the plants. It does help in small enclosed areas. Several setups have been built and shown here over the years. The best ideas involve insulated foam coolers dry ice and a water drip setup and small fans. You can have the drip setup drip onto the chunk of dry ice giving off co2 and having the fan expel the co2 into a small grown box or tent. It takes alot of co2 to get the best result so depending on your area and money you have this isnt the most cost effective but can help in a very small grow of a plant or 2. You want to have the co2 going during lights on time no need when lights are off as the plants do not process it without light.Better off with a simple co2 tank but yes this does help plants..how much depends on how well you keep it flowing.

Theory and practicality are two different things. Any excuse to play with dry ice is a good one IMO

Some info on that I've put in my Journal here> http://www.growery.org/forums/showflat.php/Number/794485 It's a good read

OK, cool, so we actually were on to something.

My grow room is below ground (my house came with an indoor swimming pool and a 3 car garage, one bay of which has an oil change pit). The pit is about 4' wide, 30' long and 6' deep, covered, with cement floor and cinder block walls. I am totally picking up a couple of pounds of dry ice and giving it a try, from OUTSIDE the pit of course