[{foo:"original foo",overrides:[{property:"foo",value:"foo override"},{property:"bar",value:"bar override"}]}]

db.collection.find({},{overrideOrOriginal:{$cond:[{$in:["foo","$overrides.property"]},{$arrayElemAt:["$overrides.value",{$indexOfArray:["$overrides.property","foo"]}]},"$foo"]}})