Continuing my debugging (and posting here in case the record of this helps someone in the future).
The installer under BIOS is now trying to execute my preseed, by adding this to my build script prior to running:
mkdir -p ./config/includes.installer
cp -v ./config/includes.binary/install/* ./config/includes.installer/
I'm not sure on the difference/relationship between those two paths. But it's starting to execute now. I'm getting an error code on one of my preseed partman early commands, something about the logger and an unterminated string. The command itself isn't bad, but at least I've got something to pursue. Still working on it.
-SG