Thanks everybody who did send me info about the RT-11 filesystem. 
I started writing a small module that could read it, and it was rather
simple to get the basics going. I created an image with arff, containing
two files, and put it onto a unused partition:

bakfull# cd rt11fs
bakfull# make load
/sbin/modload  -o rt11fs -ert11fs_lkmentry rt11fs.o
Module loaded as ID 0
bakfull# modstat
Type    Id  Off Loadaddr Size Info     Rev Module Name
VFS       0  -1 f6857000 000c f6859000   1 rt11fs
bakfull# cd ../mount_rt11fs/
bakfull# ./mount_rt11fs /dev/wd0g /mnt
bakfull# df /mnt
Filesystem  512-blocks     Used    Avail Capacity  Mounted on
/dev/wd0g        65467       22    65445     0%    /mnt
bakfull# ls /mnt
A.OUT   NYTID.C
bakfull# cd ../um
bakfull# ./a.out /mnt
bakfull# modunload -i 0

If someone wants to play with the (unfinished) code, it is available at
ftp://bakfull.campus.luth.se/rt11fs/rt11fs.tgz for anon ftp. The filesystem
isn't usable to anything else than listing files yet :-) 

I will be in Hannover next week (and the week thereafter) so if anyone 
care for a beer some evening, I won't complain :-)

I have discovered that it is _very_ simple to write a new filesystem 
for NetBSD :-)

Have fun,
	-- Ragge