This issue is squarely faced by mathematicians all the time (in fact, mathematicians and linguists are very similar in their voraciousness of pressing unrelated or novel symbols into use in extending their notatins to new sub-fields).
If a notational extension is very new, and not widely adopted, it makes sense holding off on permanently adding characters to support it -- until it is more widely established.
For historical notations, issues are different. If a modern notations has completely replaced the historical notation, it should be treated the in the same manner as archaic scripts, that is, the focus should be on what's needed or useful to support historians of the discipline. If a notation was widespread before being supplanted, that would strengthen the case for supporting it, as the likelihood that symbols will be referenced in modern contexts is that much greater.
If occasional use or reference to the historic notation can be documented, then it would be more appropriate to treat it like a rare script, or like historic additions to modern scripts, which see occasional use.
If there's known ongoing use, or documented recent citations of older notation, then it's really a case of modern use of a specialized notation and it should be treated like that.
A./
