I haven't had a chance to experiment with this yet, but my understanding is that with the right combination of BLIMP Windows application and evaluation board firmware, users will be able to program the ADV8005 serial flash memory using the BF524 USB.
With regards to your second question about needing USB for programming, it really depends on two things:
1. how much OSD/GUI data you want to store in the ADV8005 serial flash memory (it could be as little as 0MB or as much as 16MB)
2. how often you plan to update this data. (if you are able to simulate your OSD/GUI well using BLIMP, it might reduce the number of times you need to reprogram the ADV8005 serial flash memory). At some point, you may consider your OSD/GUI complete, and may not need to program the serial flash memory more than one time (before production, probably.)
I hope this helps.
-Matt