In the bright light of morning, I'm still determined to sell this beast. According to Robert Heinlein, I've followed three of his rules already! (Actually, I've already started following Mr. Sawyer's sixth rule. "Midnight Train" is well underway, with two thousand words already written and copious notes about tidally locked planets, coffee ceremonies, and Paul Krugman's paper on calculating interest rates when traveling at near-luminal speeds)
In a way, I'm lucky: the length of the piece severely limits its market, but leaves open a couple options that I'm happy with. I'm adding a further constraint: I would like to be eligible for SWFA membership. Of the Big Three SF magazines, Analog and Fantasy & Science Fiction Magazine look like my best bets. (Asimov's, whose submissions guidelines apparently are not linked to from the site itself and must be Googled, discourages works over 15k words, but does not refuse them) F&SF explicitly takes works up to 25k words, so mine fits nicely, and they're specifically asking for more science fiction.
There are relatively few other choices. Orson Scott Card's Intergalactic Medicine Show is one possibility. I'd kind of like to avoid web-only publications, for the simple reason that I rarely read them myself (with one exception). However, OSCIMS *is* one that I've read in the past, and it looks like a decent place to publish. (No, I'm really not interested in discussing OSC's politics in the comments to this post)
And... that's pretty much it. That's a distressingly short list. But given the wait times involved, it could well take a year to work through it, so I'm not going to worry about it right now.
Sadly, I do not qualify for Sheep! Magazine, published out of Trout, WV. Maybe Inspector Crandall's next case will be more appropriate.
Also, the Writer's Market website is freaking useless. I have NEVER seen a search utility so horribly broken in my life. I type in "science fiction novella", terms that I have seen for myself in their listings, and it comes up with exactly one: a listing specifically saying "no science fiction". (It also says "no detective fiction" -- it read my mind!)