It's very possible because the underground layers are defined in biomes.
You'd just insert a bedrock layer less deep or structure it any other way.
I tried and I cannot get it to work. Whatever number I change to the bedrock layer, nothing seems to happen. Hardcoded maybe?
append xpath="worldgeneration/biomes/biome[@name='snow']/layers">
<layer depth="*" blockname="terrBedrock"/>
</append>
<append xpath="worldgeneration/biomes/biome[@name='pine_forest']/layers">
<layer depth="*" blockname="terrBedrock"/>
</append>
<append xpath="worldgeneration/biomes/biome[@name='desert']/layers">
<layer depth="*" blockname="terrBedrock"/>
</append>
<append xpath="worldgeneration/biomes/biome[@name='water']/layers">
<layer depth="*" blockname="terrBedrock"/>
</append>
<append xpath="worldgeneration/biomes/biome[@name='wasteland']/layers">
<layer depth="*" blockname="terrBedrock"/>
</append>
<append xpath="worldgeneration/biomes/biome[@name='burnt_forest']/layers">
<layer depth="*" blockname="terrBedrock"/>
</append>