Hi,
Thanks for your replies expanding my knowledge.
My project however requires that a user logs in without providing the command to be executed. My final (non Jailkit specific) solution is this. The jail's /etc/passwd file refers to the single program to be executed, for example:
myuser:x:1001:1001:,,,:/home/myuser:/bin/pwd
The program could be a binary as well as script written in any scripting language.
This way, to save resources, a shell isn't loaded because it isn't needed. To have a line in /etc/passwd that refers to something other than a shell is unconventional, but as far as I know it's perfectly legal. Until today I hadn't been aware of that.