I'm not aware that the (internal) DSL modem logs anything in the system log. Of course resyncs cause a WAN disconnect, and that is logged in the syslog, but I am not sure that the modem itself logs anything.
(Edit) In a way it could, but it doesn't: A change in line status triggers several scripts in /etc/hotplug.d/dsl/, one of them for example takes care of the LED signaling of the line status. However, nothing in there logs anything about the line status into the syslog.
I am also not aware that SRA activity prompts a modem statistics value to change -- other than the line rate itself of course.
That being said, it would be trivial to check the line rates at regular intervals (this can be every minute or even more frequent) and, if they change, log time and new rate, to the syslog or to some other file. Is that what you want?
Also, as a sidenote, if you're running a Vectoring line, you should absolutely consider switching to a current snapshot. Improvements to the error handling that significantly improve line stability have not landed in a release OpenWrt version yet.