Good grief. To those calling the OP out saying it's not cheating,
Sandro has come forward and clarified this issue nearly two years ago. I made this a huge link on purpose so you don't miss it.
I can't think of a completely infallible way of telling a loss of connection apart from a combat log, but you can leverage the client itself to provide this info, as a first approach. This will be trivially defeated by enterpreneuring players, however. Will your average logger bother? Probably, this would affect mostly the incidental logger.
As a second approach, you would look at when and under which circumstances the event happened - yes, the instances are client side, but the matchmaker knows when and how and why an instance change happened; for instance, following an interdiction, etc. You can build a pattern with this.
Neither are fool proof, but this is no reason not to do it.
E: On topic, the potentially minute amount of people doing this in solo or not has no bearing on whether this issue needs solving. Fixing it (punishing it per my suggestion) for Open fixes it for Solo.
It's the principle of the matter.