Having given the matter a little more thought:
I'm not sure there is a modern version of a maxima, as there
is of a breve or a long. A good reason for the lacuna is that
there isn't much--or any--need for one, save for historical or pedagogical
reasons. I don't have quick access to a good university library
any more to look up the standard notational references.
The more reasonable approach is to assume that there is no
"modern" notation for maxima, and to just use the Baroque/Renaissance
equivalent. That is perfectly appropriate for foreseeable use.
Moreover, the modern breve and long notations are really
ugly--why use them?
Lilypond provides a perfectly reasonable way to notate
a maxima for any foreseeable purpose. That should suffice.
There is a secondary question: How does Lilypond deal with
duplex maximae and longae rests in perfect maximodus and modus?
I haven't explored the former; I think the latter is not dealt with correctly.
The issues are encountered rarely, and one
cannot expect volunteer Lilypond mavens to deal with them.
And it is always possible to force a glyph.
Be grateful for what you have--it's an amazing package!
And be grateful to the folk who maintain and extend it.