Linux: Case-INSENSITIVE Filesystem

If you're talking about case insensitivity ONLY within apps running under Mono, you can use the IOMAP environment variable to tell mono to ignore case just like Windows does.

See the details on its use here: http://www.mono-project.com/IOMap

There's a case insensitive ext3 driver, but, frankly, running such a thing in production would scare me a bit, since you're rolling your own custom kernel with patched-in drivers. (Yeah, yeah-- we did that all the time "back in the day", but that was, like, 1997...)

There's a FUSE implementation, ciopfs, that implements case insensitivity as well. That would be a lot less scary to me to use in production than a patched kernel.

You could also use a case-insensitive filesystem, like VFAT, for the /var/www folder hierarchy. I'd use a loopback mount in that case, such that you don't have to mess with your disk partitioning.

Necromancing.
Answering my own question.
This is indeed possible.
Variant 1:
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
umount /mnt/jfs/

Variant 2:
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

umount /web

reformat:

mkfs.jfs -O /dev/hda5

remount the partition

mount /dev/hda5 /web

Now run

blkid

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...