From the mailing list:
3) Disk partitioning -- I ran into three issues here:
a) The initial screen (custom-3.png) shows a completely empty disk,
which is fine; however, this led me to believe that I would need
to create an EFI System Partition (ESP) -- but the screen in
which partitions were created (custom-4.png) had no option for
a FAT filesystem or an ESP. Nonetheless, I proceeded, only to
discover that the system created a 512 MiB ESP. Creating the ESP
automatically is certainly sensible, but the lack of warning
that this would be done is confusing, at least if you know
what you're doing in general but are unfamiliar with this
installer.
b) In the preceding sequence, I specified a partition number of
"1" for my first partition, but this was taken by the ESP,
bumping the partition I specified to 2. A little more
experimenting showed that the partition number field (on
custom-4.png) was ignored for the first partition I
created; it ALWAYS showed up as partition 2, even if I set
it to 4, 10, 34, or whatever. Subsequent partition numbers
were honored, though.
As much as I have a plan to fix this it is to always have an entry for /boot/efi but allow the user to specify a boot disk, which would move it around.