Answering my own question.
This is indeed possible.
Create a .dsk-file of size X-MB (with dd), then make a JSF-filesystem on that file, with option O (OS2 compatibility - = case insensitive)
Then loop-mount that file into a desired folder.
(apt-get install jfsutils)
dd if=/dev/zero of=jfs.dsk bs=1048576 count=150
mkfs.jfs -O jfs.dsk
mkdir -p /mnt/jfs
mount /volumes/jfs.dsk /mnt/jfs -t jfs -o loop
When you install the OS, manually partition the disk, and create another partition with format JFS, which you mount to wherever you want to have it.
When everything is up and running, you can reformat that partition with (warning - this will erase all data on that partition - make sure you selected the proper one, and don't have any data on it, yet):
unmount the mounted partition
mkfs.jfs -O /dev/hda5
remount the partition
mount /dev/hda5 /web
and you get the new partition-uuid.
Now go to /etc/fstab, and replace the old uuid for /dev/hda5 with the new one.
If you're doing this wrong, see also
Welcome to emergency boot mode...