Can you use the same ODBC config to connect to the database with another application such as WinSql, MSAccess, or Excel?
That would at least cut down on the number of things that might be wrong if your ODBC config works with everything but your TCL frontend.
Also, this might be a total reach, but I have seen third party OLEDB drivers that provide built in "test" facilities you can use when setting up a new connection that do not really test the connection, but actually just test that you have completed all of the required fields! Weird and misleading, but I have a version of Sequelink that does just that and it took us a while to realize it and stop using it. I know OLE is a different beast, but you may be butting up against the same type of issue.
One other thing is I know IBM has an ODBC ping tool you could do testing with, but thats really all I know and I have never used it and dont know exactly how to get it. I would bet its in the Informix SDK though.