fwiw, JSamuel created a custom Chaos Calmer build for the BT Openreach VG3503J modem in 2015 that also provided extra DSL line parameters and version info.
Above image retrieved from:
https://forum.kitz.co.uk/index.php/topic,15918.0.html
The thread links to:
http://vanquishsolutions.co.uk/openwrt/VG3503J/
Would be nice if these extra DSL parameters could be added to a future OpenWRT too as they are useful for diagnosing line faults.