Thanks very much for that, it confirms my understanding of SIN498 and G993.2, which I had a quick read through last night following gt94sss2's post. I guess the next question is how does DLM react in the case of missing or corrupted data, or am I pushing my luck with that one? 
Section 11.2.3.7 of the VDSL2 spec tells the modems what they should be doing with these counters - which is essentially to maintain them as required by G.997.1, which itself is the generic management spec for all DSL types.
If the modem is in SHOWTIME state, then the EOC channel should be running, and messages can be sent, both requests and responses. The DSLAM ought to be able to get a request through and a working CPE modem ought to respond.
If the CPE modem is broken in that it can maintain sync, keep SHOWTIME, but fails to respond to the EOC request, then I guess it breaks the rules of transmission down the EOC channel. Section 11.2.2 gives us the timeout information for the messages, so that the command would be retried. The "Management Counter Read" command is of normal priority, and has a timeout of 800ms. After retrying a number of times, the command is abandoned.
But there appears to be no other consequences of a failure to respond. There is nothing to trigger a resync, for example.
In normal circumstances, the retry would be enough to cope with transmission faults. And serious transmission faults would likely cause a resync because of problems on the user path. But there seems little protection against a flaw that only stops the EOC response getting through.
