I absolutely, positively would not do it. For years my neighbors put weed-n-feed on their yards and of course when it rained I got the benefit (not) of their chemicals. Weed-n-feed kills broadleaf plants living in the grass. What do you think it will do do your aquarium plants?
Then there are insecticides, runoff from roads possibly treated with salt in the winter, runoff from Bubba's oil change he did on the driveway.
I love to find ways to save $$, but I'm thinking substrate is not one of them. You could pour hundreds of hours of planning and work and have the most glorious plans, but if Bubba was in the mood to change his oil, or do the weed-n-feed, or even wash the bbq and the runoff came over to your place, your plans for this beautiful tank would be as dust in the wind.