I heavely disagree here, there is significantly more lava per chunk then there is clay. Additionally finding said lava is easy ==> branch mining @ lvl 11/12. I can also combine my obsidian collection with mining. Both take more time to collect then usual because most clay will be underwater. Now if I read the minecraft forums correctly, clay will spawn like dirt/gravel/granite/etc in the future. But thats 1.8, wich is still far away for modded minecraft. Nevertheless, thats a great addition (gogo random mod add this for us mod users already
).
That said, lava in modded minecraft is a joke because of chunk loading and fluid pumps (<== both require extra mods). Getting obsidian without pumping the nether/lava lakes is more difficult and takes significantly more time then gathering dirt. Use bucket on lava, use on tank is gonna take ~1s. In that 1s you can sweep your efficiency V shovel and get 10 dirt.
Now ofcourse, we can make lava from netherrack. Netherrack can be insta mined like dirt, but this is going to require 2 magma crucible's and 10 dynamo's while still leaving a 1second gap per craft where the ignous extruder is not producing obsidian. You can go further though, you can completly automate it using 4 magma crucibles, 20 dynamo's and 1 more ignous extruder for cobblestone. Still has the 1s gap per craft though.
Now looking from this perspective, clay creation is around the level of netherrack based obsidian creation. Not bad but can be better and like I said, its a Tier 2 building block. Its not realy used for anny kind of mod based items. As such it should be easyer to get then obsidian. We would achieve that by having TE's recipy using sand instead of dirt. Or TE introducing a relatively easy way to generate dirt.