pdbedit will allow you to set the RID at account creation if the account is a normal user account (pdbedit -a -u someuser -U 5000). However, pdbedit will NOT allow you to set the RID if the account is a machine account (pdbedit -a -m -u testbox -U 1000). I am tried with and without having idmap uid and idmap gid options in the smb.conf file.
New design in 3.0.32. Closing.