As suggested in the title, I have a chance to fly to a hub on the same flight number that later goes on to London (different aircraft). I recall there is some sort of advantage for boarding priority on leg 2, given my presence on leg one. I'll be S-2. Does that give me some "super-seniority" or otherwise elevate me on the standby list? Thanks for indulging this elementary-level question.

That changed a couple of years ago. The travelnet has a chat function that you can ask questions

Long story short you are on the nonrev list in each city that you nonrev to or through so you compete with originating nonrevs in through flight cities.