I am not sure if this will work, but you might try using the ROWCOUNT 1 option on the IMPORT to only import the first row. If the first row LOB data is not too large for logging, then you may be OK.
edit: Sorry, I did not see that you already tried to import only the first row.
You might try a where clause to import some other row other than the first row to find one with a LOB less than 1GB. However, I thought that if you do not have your LOBs in a separate file (outside the IXF), then the data will be truncated at 32K.