I think you want a lithium BMS on the CAN bus, not the rs485.
The SI6048US has CAN that the REC, Batrium, or your own BMS can talk to, if the SIs are set to Lithium mode. That is what madsci has hooked up to (read back a little more in this thread, and he has more details here
https://github.com/madsci1016/SMAVenusDriver). Also further back in this thread, MurphyGuy has 2xSI6048, + ac coupled SMA sunnyboys, and lithium with REC bms on CAN bus, so that does work also.
If the SIs are in FLA mode, there are different signals on the CAN bus, I'm using lead batteries. I modified a script from python-can to listen on the CAN, label signals and publish to mqtt. Still some more signals to figure out.
https://github.com/dalklein/python-can/tree/can2mqtt
There is an rs485 bus also, that needs a piggyback board to connect to, and maybe that will allow changing settings in the SI configuration as well as requesting most any signal there is in there.