RNS 315 appears to expect a locked SD card, with the password derived from the CID. Given the unencrypted nature of SD card communication, we can simply lock a card with a bogus password and then sniff the communication, which will reveal the expected password.
The code in sdlock.py will allow manipulation of the SD card password settings.
import binascii
import sdlock
pw = binascii.unhexlify(expected_passwd)
c = sdlock.SDSPI()
c.init()
c.lock_unlock(1, pw)
print(c.acmd(13, 0, 1)) # poll extended status (7.3.2.3 Format R2)