In section 3.6.1 of the 18F46J50 datasheet the following is written:
Because VDDCORE could fall below the SRAM retention voltage while in
Deep Sleep mode, SRAM data could be lost in Deep Sleep. Exiting Deep
Sleep mode causes a POR; as a result, most Special Function Registers
will reset to their default POR values
Therefore you will need to save data using the PIC's flash self-write capability or to an external SRAM or EEPROM/flash chip. Except the two registers you mention in the question there is no reliable way of storing data in the internal PIC RAM. If you choose an external chip then obviously you will need to pay attention to its power requirements.
Depending on your reason for choosing deep sleep you might be able to find a suitable low power SPI flash part. Any non-volatile external chip could have its power controlled by your PIC, e.g. by using a FET or LDO with enable control. You could power it up to save state before sleep and restore state on wakeup, just make sure you give it enough time to complete writing before powering it down again.