-changing "Starting location" to "Location" and changing "Location X" to "X", therefore shortening the combo box
That sounds like a pretty good idea, although if you load a save you select the player from the save who you wish to control, which means it needs to be able to show most of a player's name there. I think that's the reason why I kept it long when I last redesigned the lobby (and because horizontal space wasn't a big issue).
-shortening the combo box for "Free/Computer/Locked" - I believe it takes ~40 pixels too much (when the nickname can have 16 chars), unless there's some language that has a very long word for one of these terms
Depends what your nickname is

"WWWWWWWWWWWWWWWW" only fits about half of it in the current space. Even "ABCDEFGHIJKLMNOP" doesn't quite fit. Given that we don't really need any more horizontal space at the moment I think I'll leave it as is.
-moving "Host configures players" over peacetime slider
I see no problem with that

It was placed above the players because it relates to them (and we had enough vertical space with only 8 players) but it would gain us an extra few lines if we move it.
-removing margins at top and bottom, moving the OpenGL info somewhere else (if it really has to stay visible in menus for all the time - if not the case, just move it into the Options - then make it visible only on the start screen, where it doesn't make unnecessary clutter)
The margin on the bottom could be made a bit smaller, but I really think we should keep some margin otherwise it looks quite odd (and some monitors cut off a few pixels top/bottom).
The "OpenGL info" is actually the version of the game (with the OpenGL version displayed at the end as well). I want to leave that because it's very useful to immediately see what version someone was using when he sends us a screenshot. It doesn't take up much usable space.
-maybe moving "Exit" and "Start" buttons below the PT slider as well?
Unfortunately I don't think they'll fit there (remember we need to allow a bit of extra space because some languages have longer words for certain phrases)
This way, I think there'll be enough place to include ~4 additional players on the list. I think it'd be good to make 2 spectators use 1 player line - spectator needs to display only his flag, nickname, team, maybe some little thing to change the type from "spectator" back to "player", maybe "ready" as well. So it's possible to fit 2 spectators in 1 line I think. (I assume pings don't matter since your implementation of spec mode will not require specs to be synchronized?)
I think it would look pretty ugly with two spectators per line, it wouldn't line up with the other players and the second spectator would look like he was floating off in the middle of the screen.
Do you think the text "Post message:" at the bottom could go? Is it explanatory enough to have an empty box below chat where you type messages? (IMO it is)
IMO chat could be made smaller for the default resolution, but sadly we can't do that because the minimum vertical resolution already has chat as small as we can make it. We could have a scrolling player's list if your vertical resolution is less than 768, otherwise the player's list can just take up chat area. Or, for small resolutions we could have a separate tab for player's and for chat.
On another note, we have plans for more game options (e.g. mutators, starting resources, game speed before/after PT, etc.) so I think we'll end up with two tabs for the area on the right (Map and Game Options) unless somebody has a better idea.