quentec
I also don't think its wise to have the smoke handling systems for the APU battery, require the APU to keep running.
When Boeing conceived and designed B787 some of the legacy system design was almost same as the older Boeing planes. What we have on this aircraft is not a smoke evacuation system, but an equipment cooling, by drawing air from the cabin by using a fan to blow the air through the electronic racks. This air is exhausted on ground to the outside. In flight this air which is warm, is ducted in to the cargo compartment to conserve the heat energy. A smoke detector is there only to divert this air when a smoke is detected in these ducts. In flight, by opening valves air and smoke is vented outside through the same port which is used on ground. Diff air pressure will do the job. No point in dumping the smoke in to the cargo.
This design is the same from B747 classic of 1960s design. When Boeing designed the B787 they had no idea that their battery will go up in smoke in violent fashion. They stuck to the legacy design as the battery fire was not in their mind then.
When APU shuts down, the venting also stops (on ground) as the Fan blowing the air will also stop, and there is no diff pressure. Functionally, I doubt if this vent duct can with stand a lithium overheated discharge. Now Boeing is coming out with a strong box and external venting. Details are unknown, and I am sure it will not be through the existing ducts which are very thin fiber material. It will require a re-design as well. watch this space.