The finished code from https://demonnic.com/targeted-tables/

targetedtables.lua

-- this function allows us to take a name of a variable as a string for checking later.
-- For instance, if you use getValue("demonnic.target") it will return the value at demonnic.target, or nil and an error.
local function getValue(location)
  local ok, err = pcall(loadstring("return " .. tostring(location)))
  if ok then return err end
  return nil, err
end

local meta = {}
-- return an entry from the table with the target swapped out for %s token
-- @param which the table being looked in
-- @param what the key being looked for
function meta.__index(which, what)
  local entry = which.entries[what]
  if not entry then
    return nil
  end
  local target = getValue(which.target)
  return string.format(entry, target or "DEFAULT")
end

--- create an entry in the targeted table
-- @param which the table to add the entry to
-- @param what the key to use for the entry
-- @param becomes the value to set the entry to
function meta.__newindex(which, what, becomes)
  local becomesType = type(becomes)
  if becomesType == "nil" then
    which.entries[what] = becomes
    return
  end
  if becomesType ~= "string" then
    local msg = "You must provide a string and a string only for values to a targeted table"
    printDebug(msg, true)
    return nil, msg
  end
  -- uncomment the following if you want to also ensure they include a '%s' to substitute the target in for.
  -- if '%s' is not included with the string the target won't get pushed in, but it doesn't error
  -- so that's why I left it commented out
  --[[
  if not becomes:find("%%s") then
    local msg = "You must provide a %s in the string to replace with the target"
    printDebug(msg, true)
    return nil, msg
  end
  --]]
  which.entries[what] = becomes
end

function createTargetedTable(target, entries)
  -- use an empty table if no entries provided
  entries = entries or {}
  -- do some type checking
  entriesType = type(entries)
  targetType = type(target)
  if entriesType ~= "table" then
    return nil, "createTargetedTable(target, entries): optional parameter entries as nil or table expected, got " .. entriesType
  end
  if targetType ~= "string" then
    return nil, "createTargetedTable(target, entries): target variable name as string expected, got " .. targetType
  end
  local new = {
    entries = entries,
    target = target
  }
  setmetatable(new, meta)
  return new
end