Replacing the entry in /etc/passwd works if the shell is compatible with the Maemo scripts which are run at boot. Bash 3.2 is compatible, I run it for a long time as the default shell with no problems. Bash 4 SHOULD be compatible but I haven't tested it (no need for it), you can try replacing the entries in /etc/passwd (ONLY for user, root and eventually others users that you created) but make a backup first.