DB2 Connect (or a license for DB2 Connect) is always required to connect to OS/390 DB2. That is why you cannot connect to OS/390 directly from you workstation. Interestingly, the DB2 client has the DB2 Connect code built into it, but it just needs a DB2 connnect license. DB2 ESE server for LUW comes with a 5 user DB2 Connect License.
You can setup a UNIX or Windows box as a gateway DB2 Connect server to the OS/390. Or as mentioned, you could use an existing DB2 ESE for LUW server with a 5 user license of DB2 Connect instead as a gateway. The OS/390 DB2 sub-system would be cataloged as a remote DCS database on the DB2 connect gateway, and any remote DB2 clients would see it as a DB2 LUW database on the gateway machine.
If you set a gateway machine for all connections to OS/390, you will need to figure out how many users will be connecting at one time and get the appropriate number of user licenses or get the unlimited edition.
The above applies to DB2 LUW and Connect version 8. Not sure how things are packaged in V9. The version of DB2 on OS/390 does not matter.