partitioning – Can preseed automatically install Debian system with separate boot disk

I would like to automatically install a Debian system with multiple disks. There is /dev/sda for /boot, /dev/sdb for several LVs (for /, /home, etc.), and /dev/sdc for PostgreSQL’s /pddata. I currently have a hard time understanding if and how this can be done with preseed.

FWIK partman-auto/expert_recipe can only partition single disks, so it could presumably serve for partitioning /dev/sdb (the disk with the most complex layout). The usual workaround for partitioning a second disk employs preseed/late_command, whoever since it runs commands just before the install finishes it presumably cannot serve in relation to /dev/sda nor /dev/sdb because the installer must copy files to both of these before it finishes. There is also partman/early_command, which sounds promising. However, it runs immediately before the partitioner starts, so before installation of the base system and hence before fdisk or parted are available. (At least that’s what a quick experiment has suggested.) So it’s back to the aforementioned limitations around partman, it would seem.

Am I missing something relevant here? Can preseed serve for automatically installing a Debian (Ubuntu) system with disk configurations such as mine?