As a DM I've had some success requesting an EC seat when they process upgrades (when I know I'm out of the running even after having to buy the flight late - hence no EC avail at purchase). However, the one case that I do remember the GA told me that others had already asked but she said "for a DM I will see what I can do"... And she did

As a silver? I don't think there are any strategies on full flight to score a comp upgrade, but you can always try.