I know people keep holding out hope for the ScanGaugeII, and I have one myself, but it's really not a fit.
It just does ODB-II messages -- first over the ISO9141-2 low-speed bus, and then using OBD over CAN.
I don't think it can do generic CAN message snooping, but I could be wrong. I think only does OBD queries over CAN.
You can't tell it to watch for a specific message, interpret the data this way, and display it. At least, that wasn't in my manual.
One option for display would be to grab the "true" SOC and re-purpose some part of the LEAF dash to display it.. like the Range display. Just override the display messages coming from the ECU with SOC. I don't know if you would have to intercept (reroute, drop message, insert new one), but since you were able to already display your own values, I suspect you can do it through the port already.
If you can display text on the "time to charge" display, even better.
This is actually similar to what the Tesla did with the Lotus dash. It's all CAN messages, on a separate bus than the motor/brake controls. They just re-used the LCD display are for more Roadster-specific things, last I saw.