Mapping notes:
First and foremost, you need to get UT2D into UnrealEd. To do this, first, make sure UnrealEd is closed, then open UTEditor.ini, and find the section that reads [ModPackages]. Add a line to it that reads:
ModPackages=UT2D
Then copy
\Published\CookedPC\Script\UT2D.u
to
\Unpublished\CookedPC\Script\UT2D.u
Another important thing is proper bot pathing. You need to modify UnrealEd to make bot paths based on the new jump height. Just open up UTEngine.ini, goto [UTEditor.UTUnrealEdEngine] and set ScoutClassName=UT2D.UT2DScout. Make sure you're under the UTEditor section. There are two other ScoutClassName properties, and it has to be the one in the editor section. Make sure if you ever map for regular UT3 again to change this back.
You can now start UnrealEd.
Mapping for 2D has a lot of different rules. Naturally, all maps should have the prefix 'UT2D' and should support all the UT2D gametypes. So, place UTTeamPlayerStarts and UT2DFlagBases (More on flags in a bit). Flag bases can be in Deathmatch games. They just don't have flags on them, so it's safe to include them.
Realize all characters will always be on the plane Y=0. So spawns, factories and triggers should be accessable from this 2D plane. When your done with a map, go through all the pickups and navigation points and make sure they're at exactly at Y=0. You can do this easily under the actor properties in the "Movement" section.
The camera won't always be in the level. Make sure there is always something for the camera to see to avoid the hall of mirrors effect. Chaotic is a good example of this. Tentacles at the edge of the level block players from ever seeing outside it. Make sure there's always something to look at, and don't let the camera get to such a position where the rest of the level is totally blocked out. The spectator camera is free to go anywhere, though, so don't try and stop it.
Surround the 2D gameplay plane with 2 UT2DBlockingVolumes, leaving a 64uu valley. Look at one of the included maps for an example of this. This way, players who feign death will stay on the 2D plane, and bots know they are restricted to it as well, and don't try and strafe out of it. It lets dead ragdolls through, too.
Some actors should be the UT2D version. Rule of thumb, look at an actor's child classes and see if there is a UT2D version first before placing it in the level. There are special flag bases and poweup factories so they spawn 2D versions that don't bounce out of the 2D plane when dropped. For weapon factories, you can use the normal ones, but make sure they only spawn UT2DWeaps.
Bots can swim, but barely. They have trouble moving vertically and jumping out of water. I suggest that if you have water in your map, don't require them to move vertically and give them ramps to get out.
It'd be best to put things in the foreground and background for aesthetics, even though they can't be touched. Don't put too much in the foreground, or it will get in the way of the camera.
Add vertical corridors for players to wall dodge back and forth off of and move vertically!
On the UnrealEd toolbar, click View->World Properties. In that, click on WorldInfo, and expand GameTypesSupportedOnThisMap. If you include the UT2D gametypes, then they will be cooked into the map file. This has its pros and cons. If I make a change to the UT2D code that would break your map, it continues to function the way it did when you cooked it. I think you need these for it to work on the PS3 as well. However, if you want my changes, you need to recook and redistribute your map. This shouldn't be a big problem, though, because I don't intend to work with UT2D much after this release.
Be original and have fun!
