Well thanks for the quick response! I looked around a lot before asking, but didn't come up with anything. I thought maybe I was just searching wrong.
I am a bit surprised bedrock height can't be set by any mod or world-generator. It seems like someone would have made this happen by now. The closest solution I've found is importing a superflat world into a regular world with MCedit (untested), but I have zero experience with MCedit and I was hoping for a more straightforward solution. Anyhow, thanks again man.