Change to a modifiable _ENV, whilst keeping access to existing environments.

envlist.lua

local mkenvlist_mt = {
	__index = function(self, index)
		for _, env in ipairs(self._ENV_READ_LIST) do
			local value = env[index]
			if value != nil then
				return value
			end
		end
		return nil
	end,
	__newindex = function(self, index, value)
		self._ENV_WRITE[index] = value
	end,
}
local function mkenvlist(write, reads)
	return setmetatable({ _ENV_WRITE=write, _ENV_READ_LIST=reads }, mkenvlist_mt)
end

e.g.

local foolib = {}
local _ENV = mkenvlist(foolib, { foolib, _ENV })

function bar(baz, boink)
	return baz .. boink .. baz
end

return foolib