No way to do that simply with unload. With a little work and if theres an incremental primary key or something on the table that you know you could do a column>x where x is the last record you saved the first time, then you could do a full unload in the morning and an incremental based on the last record you backed up early + 1.
Before the first save you do you could always select the table into a work table, then unload the work table but leave it in place. Then later in the day do an unload/select from the original table where the records are NOT in the work table - that would give you your incremental. This would still depend on the table structure and you ability to identify unique entities in it.
I have never used ONBAR and use only ONTAPE, but I know theres no facility to due an incremental save (level 1) for just one table. I think thats the real fight...