I’ve succeeded in getting a working battery monitor kernel module. This is basically taking the application note by Technologic Systems and spinning that into a power supply class driver that reports the voltage via sysfs.
As it happens, the battery module in collectd does not see this as a “battery”, something I’ll look at later. For now the exec plug-in works well enough. This feeds through eventually to an InfluxDB database with Grafana sitting on top.