You could stripe the memory across multiple different people who are responsible for smaller fragments. Each person can be responsible for some number of shared fragments. See your issued handbook, subsection "Surviving information storage in ancient times". (They could also just write it down, they had that.)
I would like to also add my support for some of the ideas expressed earlier in the thread regarding the questions around whether literacy negatively impacts human memory, especially in children or in studied aboriginal populations. There are even studies that show that the vividness of mental imagery is reduced in people who wear eyeglasses: http://diyhpl.us/~bryan/papers2/neuro/imagery/Refractive%20e...
