Well, I found a workaround, so I'm less concerned about an answer to my question.
I had a SD card that I had used the installer on, so I just copied the files from the SD card into the new partition on the eMMC, and that worked fine.
I now have three (somewhat) different linux installs on my eMMC. Only one boot partition though.
Since I haven't got anything that does a boot menu on the builtin screen and I haven't bought a serial-port adapter, I have to edit the extlinux.conf whenever I wish to switch to a different install.
This is exactly the place I expected to get to if the installer had the feature I was asking about.
I had a SD card that I had used the installer on, so I just copied the files from the SD card into the new partition on the eMMC, and that worked fine.
I now have three (somewhat) different linux installs on my eMMC. Only one boot partition though.
Since I haven't got anything that does a boot menu on the builtin screen and I haven't bought a serial-port adapter, I have to edit the extlinux.conf whenever I wish to switch to a different install.
This is exactly the place I expected to get to if the installer had the feature I was asking about.