It would be controversial for some here no doubt but I reckon if you have a seat reservation then you Choose to not sit in it then I think when the revenue check is undertaken, that failure to sit in your booked seat should result in fine !!
I travel on the 05:48 Kings Cross-Edinburgh very frequently, usually after finishing work sometime around midnight and hanging around in London for five-odd hours. If I buy an advance ticket I have to make a seat reservation, and I often don't get to choose my seat at the point of reserving (because the Trainsplit seat selector only works sporadically and Lumo don't seem to have one at all).
Why would I choose to sit in the suboptimal seat that the system has reserved for me, very likely next to someone else, when I could sit in a pair of seats on a non-full train and sleep comfortably?
It shouldn't be beyond people to go "oh, this seat was reserved from Kings Cross and no one is sat in it now that we're going past Stevenage - I guess it's unoccupied, I guess I will sit in it".
)