The parent gametype for this is Team Slayer, and actually not Invasion. So it seems that Bungie can enable Invasion-style spawning in any gametype via script.
You set the backfield spawns by using the bro_loc_spawn label. Here's the thing.. that label didn't ship with the original game! So Bungie does in fact have the ability to add new labels with new gametypes. You will usually be applying this label to the Hill Marker objective. You do need to be forging with the Bro Slayer gametype to get the label.
This also means that unless you forge a map to support it, no map supports Bro Slayer by default. It will have to be added to every single Forgetacular submission that makes it to matchmaking, for example, if Bungie wants to have the gametype in Community Slayer. (One would think any future DLC would have default support for it, however)
The gametype only supports Red vs Blue. Sorry, no Multibro Slayer.
Fun things you can do with Bro Slayer
The label bro_loc_spawn can be applied to ANY object, and if you set that object to Game Specific, it will only show up in Bro Slayer. This is of very limited use though, but..
You can apply it to vehicles. So say you have a Falcon, Scorpion, or Mongoose. If you apply the label to any of these vehicles, you can drive them around and people can spawn on them! On certain vehicles you get sort of a neat undercarriage view as they move along. The Scorpion's is pretty neat and could have some use for machinima, the Banshee's is pretty well done. The ghost will just give you a black screen, it seems to depend on what counts as the vehicle's "origin" point in-game. Special note: applying this label to the vehicle makes it invincible.
You can apply it to the golf ball / soccerball. The camera view works great on these objects for selecting it as a spawn, and players can easily move the ball around to move the spawn. What about a gametype where Blue team can launch the golf ball over a wall to spawn inside of Red's base? Yeah, that'd be a bit more fun and useful for CTF or Assault, but it does have some pretty good possibilities.
You can apply it to armor abilities. Apply the label to an on-map armor ability. If an enemy picks up that armor ability, you can spawn on them and assassinate them! HAUNTED ARMOR ABILITIES! Apply it to armor lock! Make a gametype where Red Team are all forced to pick up an AA and have a single life, and the single person on the other team can spawn on them. Don't stay close to each other...
You can apply it to a killball! Okay, you shouldn't apply it to a killball.
