Grass and a digging tray is a great idea, i have read allot about others doing it.. i am also growing grass in a tray. just watch really close at the directions on the box on the small animal grass. i wanted to buy one but it says you have to grow the grass outside the cage and just cut off the grass once it is a few inches tall. the stuff it grows in is not good for them. but the box i was looking at is a little different than the one you posted.
actually a lot of animals eat soil because it carries a lot of minerals that they can't get elsewhere. if you're feeding your rat a complete diet that shouldn't be a problem, but animals eat dirt all the time and it's not bad for them at all (some bacteria thrive in soil, but i doubt they would be present in home-grown those little grass kits).