I've just remembered that this wishlist exists... Here I've copied our server's current layout as motivational example:
$ lsblk
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
sda 8:0 0 186.3G 0 disk
└─sda1 8:1 0 186.3G 0 part
├─vg_system-lv_root (dm-0) 254:0 0 168.7G 0 lvm /
└─vg_system-lv_swap (dm-1) 254:1 0 17.6G 0 lvm [SWAP]
sdb 8:16 0 2T 0 disk
├─vg_data-lv_wal (dm-2) 254:2 0 20G 0 lvm /media/wal
└─vg_data-lv_data (dm-3) 254:3 0 912.6G 0 lvm /media/postgresql
There *must* be sba1, because before that partition (which is LVM PV) there is free space for GRUB to be installed.
For other drive, sdb, there is no need to have sdb1 partition, it's directly a LVM PV, which can be resized more easily compared to being on a partition. But d-i does not allow users to do that in preseed.
sdb drive setup is currenly done with post-installation script in our case...