"Over the transom" refers to an unsolicited manuscript sent by an aspiring author to an editor or publisher. By mailing something the editor didn't ask for, you're slipping it in not under the door (a complete manuscript is too thick to fit) but over the transom, that tippy window-like thing above older doors that helped with ventilation in the pre-AC era.
Much con talk is over-the-transom talk: unsolicited opinions. Watching a movie or performance with this uber-geek crowd, one is treated to an nonstop stream of trivia, obscure reference commentary, witty one-liners and snide remarks — a Media Sensation's paradise. Or not, if you're trying to hear the actual dialogue. Thanks to this practice, CONvergence's opening ceremonies degenerated into an incomprehensible hecklefest as audience members attempted to one-up the emcee and each other.
Why, you may ask, do people feel compelled to do this? Why impose your cleverness on everyone present? Well, because you can. A sci-fi con may be the only place in the galaxy, other than his own living room, where a fan can feel assured that not only will others welcome his well-timed quotation from the Star Trek episode where they encounter the Roman gladiator civilization ("Bread and Circuses," episode #43, stardate 4040.7, original air date 3/15/68), they'll actually get the joke. And they won't beat him up afterward; if anything, he'll gain status.
Don't underestimate the outsiderness of this crowd; the very name of the local sci-fi club is Misfits (the Minnesota Society for Interest in Science Fiction and Fantasy). These are people who feel their "manuscripts" are seldom solicited in daily life, so the prospect of acceptance by kindred spirits is a huge incentive to offer up everything they've got. Sometimes submitting over the transom is the only way to get your voice heard.
Epilogue: I ended up leaving the opening ceremonies early because I couldn't stand the heckling. To me, it's rude. In my own unsolicited opinion, it's one thing to whisper an amusing aside to a friend; it's quite another to repeatedly interrupt a public event. Does this make me an Evil Oppressor, She Who Stifles Others' Voices? No. I didn't stifle anybody; I merely expressed myself, and did so in a manner that inconvenienced no one. That's what these conventions are all about: doing your own thing, no matter how bizarre.