I can't recall ever having seen "a honest" or similar in written text.

On a similar note, what is the rule for abbreviations where the first letter is a consonant but itself starts with a vowel sound. For example, in speech I would say "a Member of Parliament" but shorten it to "an MP". I hesitate when I come to write a/an MP as I feel the reader may mentally fill in the missing words implied by the abbreviation.

DC