(GLua) Something dirty I guess. This is supposed to let you hide players from SUI scoreboard (and cloak them) based on clientside actions.

sh_cloakhax.lua

-- Courtesy of /gh/Cheatoid
-- Updates: https://gist.github.com/d29fc5dd6ea9819295d2c87098b7bfb6
-- Totally untested. But, it is supposed to let you hide players from SUI scoreboard (and cloak them) based on clientside actions.
-- Needless shall I have to tell you again, this *will* be abused by haxxors/skids... But, you will just need to live with that I guess.
-- Whenever you want to toggle the state, simply call the hax_ToggleCloak() function on clientside.

local PLAYER = FindMetaTable'Player'
AccessorFunc(PLAYER, 'm_bHideOnScoreboard', 'HideOnScoreboard', FORCE_BOOL)
local msgID = 'hax_togglecloak'

if SERVER then
  util.AddNetworkString(msgID)
  if ULib then
    hook.Add('PlayerCloakStateChanged', 'hax_ULibCloak', function(ply, newState)
      ULib.invisible(ply, newState, 0xFF) -- cloak the player using ULib function
    end)
  end
  function PLAYER:hax_ToggleCloak()
    if not net.Start(msgID) then return end
    ply:SetHideOnScoreboard(not ply:GetHideOnScoreboard())
    local newState = ply:GetHideOnScoreboard()
    net.WriteEntity(ply)
    net.WriteBit(newState)
    net.Broadcast()
    hook.Run('PlayerCloakStateChanged', ply, newState)
  end
else -- CLIENT
  function hax_ToggleCloak()
    if net.Start(msgID) then net.SendToServer() end
  end
  local hax = vgui.GetControlTable'suiscoreplayerrow'
  if istable(hax) then -- SUI scoreboard's player_row
    hax.RealPaint = hax.RealPaint or hax.Paint
    function hax.Paint(...)
      local ply = self.Player
      if IsValid(ply) and ply:GetHideOnScoreboard() then return end
      return hax.RealPaint(...)
    end
  end
end

net.Receive(msgID, function(_, ply)
  if SERVER then
    ply:hax_ToggleCloak()
  else -- CLIENT
    ply = net.ReadEntity()
    if IsValid(ply) then -- may be invalid due to PVS
      ply:SetHideOnScoreboard(net.ReadBit() == 1)
    end
  end
end)