PlanetsCentral

If I understand this correctly, you would send out initial result files, and if there's some no-shows, scrap those along with the received turns? This probably gives a very bad user experience if someone receives an awesome starting location, plans their turn, invests some time and love, and then it turns out all was for nothing.

No, that's entirely not what I was suggesting.
I was suggesting that all the players that submitted turns stay in the game, that their turns are still valid. Only the slots for the players that didn't submit their first turn are recycled and put up for joining.
I shouldn't have to say it, but once the game has started (in that turns have been handed out), players shouldn't be allowed to quit one race and rejoin the same game as another race. Hopefully nobody would do that, but better to have it hardcoded than rely on honesty.

It had been suggested that the host periodically send out emails "are you still interested"? I've been playing with this idea in my mind, but didn't end up at a solution that wholly satisfied me.

When I first joined PlanetsCentral, I missed the first 10 turns of my first game because the game-has-started email(s) went into my spam folder (probably fixed because of your SPF changes). If the user doesn't get the real game-starting email they likely wouldn't get your are-you-still-interested email either.

if the game is about to start, everyone whose join was more than X days (X=14?) ago receives an email. If they don't confirm within 7 days, they are dropped. (Possibly with an option: I have played >20 turns during last 6 months, don't bother me next time.) Game starts if we have a confirmation from everyone that is <X days old.

This kind of solution would fix the problem I think. Fixing the problem before the game actually starts is preferable to after it started. I would personally be less generous with the 7 days (both opt-in and first turn), but the principle is sound.
Note of course that your solution doesn't preclude the original situation still happening (player says "yes I'll play" and then fails to submit first turn) but should greatly reduce it.