The variables you set work as a simple text search and replace option. So, anywhere Hiawtha finds 'MonitorService', it will replace it with '<ip0>, <ip1>, ..., <ip18>'. The BanlistMask, LogfileMask and RequestLimitMask settings have the same value pattern: (allow|deny) <ip-address>[/netmask]. Use that also in the value for MonitorService.
set MonitorService = allow <ip0>, allow <ip1>, ..., allow <ip18>, deny all