test_delegate.rb

require 'delegate'
require 'ostruct'

class MyProxyClass < SimpleDelegator
  def initialize(env)
    super(OpenStruct.new(env))
  end

  def method_missing(method, *args)
    super
  rescue NoMethodError
    super(method.upcase, *args)
  rescue NoMethodError
    nil
  end
end

env_proxy = MyProxyClass.new(ENV)
p env_proxy.USER
p env_proxy.user
p env_proxy.unknown_environment_variable

$ ruby test_delegate.rb
"usr0600121"
"usr0600121"
test_delegate.rb:12:in `rescue in method_missing': undefined method `UNKNOWN_ENVIRONMENT_VARIABLE' for #<MyProxyClass:0x007fe2120247b0> (NoMethodError)
        from test_delegate.rb:10:in `method_missing'
        from test_delegate.rb:19:in `<main>'