I suggest the “Librarian” could have a way for users to save lists of articles/files that is relevant to certain user-defined keywords, so every time the user logs in he/she will be presented a summary of articles/ saved searches, and this also ensures that those articles/files will not be deleted once the SD card is full. It is like some kind of subscription, and it provide a way for content providers to connect with listeners. (of course they can also browse the generic file tree or do regular searches)
I am not sure if this is already a feature, for I only have seen the preview at librarian.outernet.is. Now that outernet suffers from simplex and low-bandwidth, and trades with the cost of a cell phone service (as 2G/GPRS is duplex and faster at around 56kbps, and is perhaps more prevalent than the access to clean water as of now), so if you add reliability into your service, it may get a better receptance.