Any thoughts on getting a 2A or 4A 12V battery maintainer that auto-switches between supply input of 120V or 240V as needed, and splice it into the L1 and L2/N wires, as close as possible to the J1772 socket?
This would provide a charge or maintenance boost as needed to the 12V battery, any time the Leaf is actively charging via the L1 or L2 charger. This should keep the aux battery charged under normal conditions, although there still may be issues with extremely infrequent vehicle use.
The hardest part of the process would be making a clean splice into what is likely 8 (or maybe even 6) gauge stranded wires. The "intelligence" of the charging system is done through the other three connectors in the socket. The L1 wire is always 120V AC, the L2/N is either 120V AC, out of phase with L1, or neutral, depending on what is supplied to the EVSE, and both conductors are disconnected until the vehicle requests charge. And tieing in a device designed to use less than an amp to cables carring up to 30A is safe enough, as long as the connection is properly fused.
Or am I missing something?