If anybody is interested in this, I got the desired data - CAP interface a specific client is connected to - from the Mikrotik tracker component using node-red, which I use for nearly all my automations.
This is the basic flow:
[{"id":"1904c3da.bd901c","type":"api-current-state","z":"d76343fe.f55e8","name":"","server":"1095198d.1bc276","version":1,"outputs":1,"halt_if":"","halt_if_type":"str","halt_if_compare":"is","override_topic":false,"entity_id":"device_tracker.my_device","state_type":"str","state_location":"payload","override_payload":"msg","entity_location":"data","override_data":"msg","blockInputOverrides":false,"x":340,"y":1200,"wires":[["ec5d5ffa.794c3"]]},{"id":"86b405b.780adf8","type":"inject","z":"d76343fe.f55e8","name":"","topic":"","payload":"","payloadType":"date","repeat":"","crontab":"","once":false,"onceDelay":0.1,"x":100,"y":1200,"wires":[["1904c3da.bd901c"]]},{"id":"ec5d5ffa.794c3","type":"debug","z":"d76343fe.f55e8","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"data.attributes.interface","targetType":"msg","x":640,"y":1200,"wires":[]},{"id":"1095198d.1bc276","type":"server","z":"","name":"Home Assistant","legacy":false,"hassio":false,"rejectUnauthorizedCerts":true,"ha_boolean":"y|yes|true|on|home|open","connectionDelay":true,"cacheJson":true}]