Actually, if you do a google search for "kitty litter substrate" you will find that quite a few people use it in planted tanks with great success. Of course, you need to make sure that you get just plain litter with no perfumes or additives. The pure stuff just baked clay, similar (but not identical) to laterite substrates, and used just like laterite. But the quality of the clay varies greatly from brand to brand, so you have to be careful.
One popular brand used for substrate is Hartz PH5, which I found at a local dollar store for $1.00 for a 10 pound bag. Although I have not tried it, I have been tempted to do so. According to what I have read the Hartz PH5 remains solid when submersed. Another one that people often use is called Wal-Mart Special Kitty.
The subject of using kitty litter is contoversial among aquarists, but is not uncommon at all. The kitty litter has a high cation exchange capacity, which aids in making nutrients available to plant roots. If you decide to try it, let us know how it turns out.
...AV