Can articles (a, an, the) be used as substitutes for pronoun to promote gender-fair language? I think the answer is definitely yes, but only in the case of general statements that don't legally or technically involve or require clearly distinguishing between the gender of the parties involved. Adequate care should also be taken to avoid overgeneralizations that can result in confusion on who's who and what's what.
In the case of your first example, for instance, the construction using the article "a" instead of the gender-specific "his" will become clearer and more specific by adding the phrase "in reply" or "in response," as follows: "Within 15 days from receipt of the appellant's memorandum, the appellee may file a memorandum in response (or in reply)." This will give a sense of specificity and distinctiveness to the second use of the word "memorandum" (the appelle's), which may sound like an orphan-word all by its lonesome with no specific correlation to the appellant's memorandum.
In the case of your second example, the construction using the article "an" instead of the gender-specific "his" will become clearer and more distinctive by using "having invoked the right" and not simply the too-general-sounding "an invocation," as follows: "Merely testifying does not render the witness immune from prosecution notwithstanding having invoked the right against self-incrimination."
