A6 is nc on PC27, so yes you could run a wire.
NRD is available on PA29 which they seem to be using for a CS for an SD card.
A5 is on PC26, which is a PWM or another SPI slave select, but as you say it's shorted to PA29(NRD). What possible reason could there be for that?
But these two pins aren't used for any other SMC function so as long as you disable both PC26 and PA29 from being used by other things it should be OK wouldn't it?
Either way it's obvious that external memory was not a priority in the design.
Rob