JMP support found the problem. I copied the connection string from the New SQL Query() script from Query Builder. The Create database connection() function doesn't allow the 'ODBC:' part of the DSN definition. So, the JSL script needs to look like:
names default to here(1);
// trying to create the db connection and then insert into the Query
dbc1 = Create database Connection("DSN=SQLplus 64-bit ODBC Connection;");
New SQL Query(
Connection( dbc1 ),
CustomSQL( combined )
) << Run Foreground();