The example given is not parenthetical:
(i) I need a (memorable) idiom.
A parenthesis is a remark which you insert into the middle of a sentence as if you are interrupting yourself. A parenthesis contributes to the meaning of the sentence but interrupts and stands outside its syntax. In writing, we typically use curved brackets, dashes, or commas to mark a parenthesis.
The syntax of the example sentence is not interrupted by the word memorable. Instead, the word memorable functions as an adjective modifying idiom. Consequently, the pronunciation rule applies to the word memorable and the article to use is a.
Compare this variation:
(ii) I need an (well, if I need anything at all) idiom.
Not an example of great writing, to be sure. But it shows how a parenthesis interrupts and stands outside the syntax of a sentence. The phrase “well, if I need anything at all” is not part of the noun phrase “an idiom”. The pronunciation rule still applies, but it applies to the word idiom and the article to use is an. This is true even though you would not normally pair an with well. You would, for instance, say:
(iii) I need a well known idiom.
The difference is that well is parenthetical only in example (ii) above.