My seat selection strategy when flying long haul economy (as a program elite) is to try to ensure there will be an empty seat next to me. This is pretty much equivalent to real PE if it's in the extra legroom E+/MCE/Comfort forward section of coach. I'll select the last row of that section in an aisle seat of the middle bank. Since non-elites must pay a hefty supplement for these seats, it's likely anyone else will be an elite, often awaiting an at-the-gate upgrade to business. It's unlikely anyone would voluntarily select the middle seat unless the flight was 100% (and being the extra leg room section, agents will only select these seats for those without advance seat selection if no other regular seats are available).
So my luck has been pretty good over the years. Last month on an AA 767 from PHL-MAN the person who had the middle seat in the weeks prior must have been upgraded so the middle seat remained empty for the flight. (An added advantage on this aircraft is that this MCE section was a discrete 2-row mini-cabin so it had the feel of exclusivity (and both forward coach washrooms at the rear were always available since most coach flyers in the back used the rear washrooms) just as AA's 777s used to have before adding PE. Free drinks were an added benefit.
One of the best benefits of upper tier elite status should be blocking adjacent seats until really needed to accommodate full flights.