I took a quick look.
It looks like a number of the functions needed to get it working (or at least partially working) are available in classic.
I can understand why this is such a large project.
UnitInBattleground
GetBattlefieldEstimatedWaitTime
GetBattlefieldFlagPosition
GetBattlefieldInstanceExpiration
GetBattlefieldInstanceInfo GetBattlefieldInstanceRunTime
GetBattlefieldPortExpiration
GetBattlefieldScore
GetBattlefieldStatData
GetBattlefieldStatInfo
GetBattlefieldStatus
GetBattlefieldTeamInfo
GetBattlefieldTimeWaited
GetBattlefieldWinner
GetBattlegroundInfo
GetBattlegroundPoints
Source: https://wow.gamepedia.com/Global_functions/Classic
I can understand why maps in instances are not working. I do not believe the API for dungeon maps was made available until much later on. But PVP it was, since I played back in vanilla and had a few addons that modified battleground maps.