The format of the 64 byte buffers is pretty simple, and can be determined without much difficulty by looking at the firmware source code. The important fields are:
Bytes 0..51: CAN messages (see below for details). [IN/OUT]
Byte 52: Number of CAN messages to send. I have only tried sending a single message at a time. [OUT]
Byte 55: Transmit Error Counter. [IN]
Byte 56: Receive Error Counter. [IN]
Byte 57: CANSTAT register - see datasheet for details. [IN]
Byte 58: CANCTRL register - see datasheet for details. [OUT]
Byte 60: SPI command. For incoming messages, this echos back the command that was sent. Commands are 0xC0 (CAN reset), 0x03 (read register), 0x02 (write register), 0
x80 (RTS - ?), 0xB0 (RD Status - ?) and 0xD0 (Read Firmware Version). [IN/OUT]
Byte 61: Register. Set to the register to read or write. The response to a read request contains the same value. [IN/OUT]
Byte 62: Data. The value to be written to a register for outgoing messages, or retrieved from a register for incoming messages. [IN/OUT]
CAN message format
Byte 0: Most significant bit (bit 7) indicates the presence of a CAN message. Bit 5 indicates an extended (29 bit) identifier. Bit 4 indicates a remote transmission request (RTR). Bits 0-3 contain the data length.
[Standard ID] Bytes 1-2: Byte 1 and bits 5-7 of byte 2 contain the identifier, big endian.
[Extended ID] Bytes 1-4: From most significant to least significant, the message ID is in byte 1, byte 2 (bits 5-7), byte 2 (bits 0-1), byte 3, and byte 4.
Next [len] bytes: If this is not an RTR message, the content of the message follows (up to 8 bytes).