dpbredsox,
Yes, change your ONCONFIG file LTAPEDEV parameter to point to the absolute path of a file or a symbolic link pointing to a file (more on that in the next paragraph). The file should already exist and be 0 bytes in length when you start logging to it. You should also probably put the maximum file size for you OS in LTAPESIZE or Informix will error in trying to write past the OS's filesize limitations.
Also be aware that once you have filled up the file to the OS size limit you will be prompted to enter another 'tape' into the device. If you used an actual file name in the LTAPEDEV, you should copy the existing file to another name/location, zero out the current file and logging can continue. The symbolic link method is better since you can just point the link to another 0 byte file without switching file names.
Be aware these file obviously need to be backed up to tape somehow if you are interested in restoring after a disk-crash. Also consider this will take longer than usual since you will first have to restore the files from tape, and then get Informix to read these files.