I don't think it has anything to do with DIY or pressurized. The idea is that the reactor will allow you to utilize all the co2 you produce instead of wasting it when it reaches the surface. Yes you can just run it through the intake side of your filter, but that can be damaging. The reactor is truely the best route to go.
I have an in-tank, powerhead-driven reactor that I bought from AquariumPlant.com. I want to get it out of my tank though, so I'm going to build an inline reactor...and I've chosen this design. I like the clean-out fitting on this design but you may like this simpler design better.
I am setting up a new tank and will be doing the same thing. I'm not sure I've seen the design you're talking about, but I've seen some ideas around on various sites. I have a 10 lb pressurized CO2 cylinder w/ MMM regulator, needle valve, check valve and bubble counter. I decided it might be fun to do a DIY reactor, so... I started looking around for something I could use. Then I found a cylindrical plastic bottle that currently has plant food (which I will find a new home for). The approximate dimensions are: 12"L x 3" Diameter. I will need to silicone seal the lid so no CO2 can escape, but I suppose the best way to do this would be to run a tube from the top of the bottle to the powerhead/tank. The other line should go into the side of the bottle, down below the bio-balls, of which will fill the bottle about 3/4 of the way. For tubing, use the CO2-proof stuff, or whatever they call it -- its cheap and if you have to buy tubing anyway, buy this stuff. I saw some on azgardens.com and I'm sure it can be found elsewhere. Sorry for the long post and for rambling -- this is more or less my thoughts outloud, but hopefully they will be helpful to others wanting to do the same type of thing. Cheers,