With no SS there is no way to sync with the master unless you can detect the inter-frame idle.
As you have a 21mS idle period I would run MOSI to an interrupt pin and time since the last edge, after a reasonable time (say 10/15mS) you can assume that you are in sync and the next 14 bytes are good.
AFAIK without timing or SS it is not possible to detect the start of a frame with SPI.
Rob