I have no background in literary criticism (I was a history and artifacts classicist more than a literary classicist) and don't much read slash because I have too many other things piled up to ready already, but I get the distinct impression that many of the people running amok with the term and its derivatives are operating from a limited critical vocabulary, for one reason or another. Because of this, they take the term and extrapolate, extend, and extrude it everywhere they run into anything that happens to remind them in any way at all of slash, whatever that connection might happen to be.
If, for example, Billy Budd is deemed to have slashiness, how about Lolita? Sir Gawain and the Green Knight? Absalom, Absalom? Anna Karenina? Jane Eyre? Wuthering Heights? The Iliad? I could go on and on, spinning thinner and thinner threads of supposed connection, based on reasons people who have posted above have suggested the terms slash/slashy/slashiness might be applied to a work of fiction. None of these usages tells me anything other than "This work has something that I, as a regular reader of slash fanfic, find appealing." This sounds to me like a case of metonymy* with bells on; if you're using it as a shorthand, it might be helpful to unpack the thought a bit, so people will know why it rings those bells with you, because reasons which are implied but not explained may be misunderstood.**
Clear definitions are good, yes?

Also, in a world where, in my lifetime, a group of human relationships have managed to go from being The Love That Dare Not Say Its Name to The Love That Comes Into My Office To Show Me The Pictures From Its Wedding (which is not yet legal in this state but maybe someday...) it would be nice to avoid putting people back into a box, even if it's a sympathetic box.
*Which is the container, and which is the thing contained?

**If you didn't realize you were doing it, you might want to stop and consider that what you think you are saying may not be what other people are hearing. I'm just sayin'.