I dont think that when you can code (i cant code) access to the hardware would be required. The DSL status page in the openwrt overview already lists many things "/etc/init.d/dsl_control status" also do. But "/etc/init.d/dsl_control status" also lists the firmware version.
So just also pull the firmware version information from the same source every other DSL information is been pulled off. That would be everything.
This patch here looks already like if its doing everything and even more: https://vanquishsolutions.co.uk/openwrt/VG3503J/lantiq-dsl-drivers-to-v4.16.2.4.patch.txt
I got the patch from here: For Developers: Please enable displaying the Lantiq DSL firmware version