Is the language that consists of machine configurations whose language is a subset of even palindromes semi-decidable?
What you have shown is that the language of Turing machines accepting at least one even palindrome is semi-decidable. However, your question is interested in the language of Turing machines accepting only even palindromes, which is rather different. The complement of this language is the language of Turing machines accepting at least one word which isn't an even palindrome, and so the complement is semi-decidable according to your argument. This still leaves open the question of whether the original language is semi-decidable (and so decidable).