I don't think equal spotlight distribution is feasible on large games, for a variety of reasons. I mean, no one should be excluded from having their turn in the spotlight if it comes along, but you're never going to be able to ensure every one of 300 players has a chance to shine... and if you do, you're going to run into "you got a chance to shine once, a year and a half ago, so you can't go on plots anymore", which is a surefire way to burn out otherwise active players (who are the ones who stir up RP when you aren't GM'ing).
It's hard even in individual scenes, honestly; when I GM scenes, I try to ensure there's something for everyone to do -- everyone gets an opportunity to influence the outcome, gets a chance to let their skills shine. And that can be difficult when you have a group of seven people and three hours to run an adventure for them, but still possible; it's not really possible when you're running a giant mass battle scene for 40 people and don't want it to take multiple days.
I think what you can aim for is "everyone has opportunities to influence the game world, even if it's part of a larger action". And if someone comes up with an interesting idea that fits the game world and has a chance of letting some new folks have the spotlight, be prepared to run with it.
I think the best you can do when you are dealing with 300-some players is seed storylines in 3 or 4 different places, and try to nudge people together to act on it. But if someone doesn't act on it (or misinterprets a clue/vision and heads off in some other wild direction, which has happened), the people who do act on it are likely to be the ones who play a role in resolving that plotline. Or the people who pick up the clues and run with them. But everyone should have the opportunity to do such.
That said, there is a tool on Arx which returns a list of characters who've never had staff GM attention, which gets used to try to seed new storylines to people who might otherwise be struggling to find a path to involvement. I think that's a very useful tool to have, since it helps you find the people who might otherwise be just doing slice-of-life RP and wondering how to get involved.
This is, mind you, just my $0.02 (plus state sales tax where applicable).