>I think there is a trick to launch this tiff application over
> L4/fiasco, so the kernel can permit the application to create a file. 

Per default, there's no writable directory available.
You can use tmpfs. Add the following in your application's Makefile:
REQUIRES_LIBS += libmount libl4revfs-fs-tmpfs 

Then add a fstab file, with this content (usual syntax):
/     /tmp    tmpfs defaults

The last thing to do is to tell your application where to find the fstab
file. You need a config file for that, with this content:

L4.default_loader:start({}, "rom/tiff_test2", { FSTAB_FILE = "rom/fstab" });

Do not forget to change your module.list entry, add ned and put
"roottask moe rom/config.cfg" there instead of the --init version.

Now you should be able to write in /tmp in your application.