The Meaning of Life, the Universe, and Everything.
Fire's Clay Spawn is a mod I made to spawn clay underground like an ore. The clay can be set to spawn at the same rate and layers as Coal, Iron(default), Gold, Diamond, Redstone, Lapis, and Emerald. You can also manually override the maximum layer it generates at, the minimum layer it generates at, and the ore vein size. It also supports generation in styles similar to ores from Forestry and UnLogic II. I plan to add support for generating in styles similar to ores from more mods as well. All of these settings can be applied to Hardened Clay generation as well. Also, the mod can be set to generate Stained Hardened Clay, adding a bit of color to your mines.
Retrogen can be achieved using SimpleRetrogen by cpw. Just add "the_fireplace.clayspawn.worldgen.WorldGeneratorClay" to the config.
For those of you saying this is a duplicate of Pam's Clay Spawn:
First of all, this mod is a lot more configurable than Pam's Clay Spawn. On mine, you can manually set the highest layer, lowest layer, and vein size, or input the name of a vanilla ore and some other modded ores to get the vein size and/or highest layer and/or lowest layer from. And you can generate Stained Hardened Clay.