not sure about what you call a Sysback. Are you referring to System backup, Informix instance backup or anything else ?
If Informix instance backup, both ontape and onbar can write to a file. For ontape, the device on declare in the TAPEDEV onconfig parameter only has to be already existing, have 660 permissions and belong to informix:informix
Same principle for onbar.
Is this the answer to your question or am I totally out of it ?