I think it’s fine to request the single-stroke caesura as its own thing, rather than as an alternate. I’m not sure it’s any more of an alternate than, say, the bold caesura or the curved one. What I suggest you do is add an issue on GitHub requesting the addition of the single-stroke caesura (which you do here), and we’ll consider it (though not for SMuFL 1.3, which is done and more or less dusted at this point). Please include one or two citations for where the symbol appears in published music if possible.