Right now, I have GregTech's ash as the primary ash with the AlmostUnified mod, but the tool breakdown still gives me ash from the Embers mod (I didn't just look in the JEI, I actually tried it). Could you make that configurable by any chance? That would be much appreciated!

The cinder plinth now uses the tag preferences config in version 1.4.1

You can change the output to another type of ash by putting the id of that ash in the itemPreferences config. However the cinder plinth is still hardcoded to output embers ash so I'll leave this issue open until I add a way to modify its output.