So you need to find an enhancement/exit/BAdi triggered when opening a new node (a where-used on message 027 of class 14 ? SAPMSSYD ? or if unlucky this is coded in SAPGui and not much hope ?) , need to be able to identify if SAPLPLF is the opener (?) else count current number of nodes (TH_USER_INFO) and raise an error if only one free node remains available, and close or not open the new node.
Good luck,
Raymond