Thank you for pointing me in the right direction.
This worked on my XE300-EP06-E (v 4.3.18):
root@GL-XE300:~# curl -H 'glinet: 1' -s http://127.0.0.1/rpc -d '{"method":"call","params":["","system","get_status",{}]}' | jq '.result.system.mcu.charging_status'
0
root@GL-XE300:~# curl -H 'glinet: 1' -s http://127.0.0.1/rpc -d '{"method":"call","params":["","system","get_status",{}]}' | jq '.result.system.mcu.charge_percent'
51
This also worked, without the need to fetch it from the rpc endpoint:
root@GL-XE300:~# ubus call mcu status
{
"charge_cnt": "0",
"temperature": "37.3",
"charge_percent": "33",
"charging_status": "0"
}