[zork]

I think I got it!!! (But it works only out of combat atm. I need a secure way to apply the unit attribute (I think...))

--UFM:UnitAdded
function UFM:UnitAdded(nameplate,unit)
  self.unit = unit
  self.inVehicle = UnitInVehicle(unit)
  self:SetAttribute("unit", unit)
  self:UpdateAllElements("NAME_PLATE_UNIT_ADDED")
end
 
--UFM:UnitRemoved
function UFM:UnitRemoved(nameplate,unit)
  self.unit = nil
  self.inVehicle = nil
  self:SetAttribute("unit", unit)
  self:UpdateAllElements("NAME_PLATE_UNIT_REMOVED")
end

Full solution (all you need is your own style function)

------------------------------------------------------------------------------
------------------------------------------------------------------------------
------------------------------------------------------------------------------
-- NAMEPLATE TEST
------------------------------------------------------------------------------
------------------------------------------------------------------------------
------------------------------------------------------------------------------
 
--register focus
oUF:RegisterStyle(A.."NamePlateStyle", CreateNamePlateStyle)
oUF:SetActiveStyle(A.."NamePlateStyle")
 
local W = CreateFrame("Frame") --worker
local UFM = {} --unit frame mixin
local C_NamePlate = C_NamePlate
 
-----------------------------
-- Hide Blizzard
-----------------------------
 
function W:UpdateNamePlateOptions(...)
  print("UpdateNamePlateOptions",...)
end
 
--disable blizzard nameplates
NamePlateDriverFrame:UnregisterAllEvents()
NamePlateDriverFrame:Hide()
NamePlateDriverFrame.UpdateNamePlateOptions = W.UpdateNamePlateOptions
 
-----------------------------
-- Worker
-----------------------------
 
function W:NAME_PLATE_UNIT_ADDED(unit)
  local nameplate = C_NamePlate.GetNamePlateForUnit(unit)
  if not nameplate.unitFrame then
    local unitFrame = oUF:Spawn(unit, A..nameplate:GetName())
    unitFrame:SetParent(nameplate)
    unitFrame:ClearAllPoints()
    unitFrame:SetPoint("CENTER")
    nameplate.unitFrame = unitFrame
    Mixin(unitFrame, UFM)
  end
  nameplate.unitFrame:UnitAdded(nameplate,unit)
end
 
function W:NAME_PLATE_UNIT_REMOVED(unit)
  local nameplate = C_NamePlate.GetNamePlateForUnit(unit)
  nameplate.unitFrame:UnitRemoved(nameplate,unit)
end
 
function W:OnEvent(event,...)
  self[event](event,...)
end
 
W:SetScript("OnEvent", W.OnEvent)
 
W:RegisterEvent("NAME_PLATE_UNIT_ADDED")
W:RegisterEvent("NAME_PLATE_UNIT_REMOVED")
 
-----------------------------
-- Unit Frame Mixin
-----------------------------
 
function UFM:UnitAdded(nameplate,unit)
  self.unit = unit
  self.inVehicle = UnitInVehicle(unit)
  self:SetAttribute("unit", unit)
  self:UpdateAllElements("NAME_PLATE_UNIT_ADDED")
end
 
function UFM:UnitRemoved(nameplate,unit)
  self.unit = nil
  self.inVehicle = nil
  self:SetAttribute("unit", unit)
  self:UpdateAllElements("NAME_PLATE_UNIT_REMOVED")
end

I either need a spawn that does not need secure environment or I need a way to spawn/change the unit attribute securely.