Well, this morning I found out! I took him out for a dawn ride, and we encountered a Terrifying Enormous Horse-Eating Plastic Bag. Dubbin didn't just startle; he did a full-on panicky-leap-backward-and-skip-round-in-circles-away thing. There may have even been a little half-rear or two in there, I'm not sure.
And here's the amazing bit: I didn't fall off!
...Well, not entirely, anyway. One foot stayed more-or-less in the stirrup; the other one came out, and I spent a fair proportion of the panic-and-skip-backward-in-circles thing hanging half off one side, gripping the reins for dear life with one hand and heaven-only-knows-what with the other, trying to yell "WHOA!" soothingly while thinking OMG I'm going to die! and thinking that the sand of the arena looked a lot further away right then than it had a moment before. The whole process felt like it lasted for an eternity, but in reality it was probably only ten seconds or less of Dubbin being all Eeek! Getitawayfromme! while bouncing backward in circles.
When it all settled down, Dubbin was calm and my adrenaline levels had returned to normal, my main thought was My god! I actually stayed on the horse through that! I'm sure it'd all be very ho-hum and day-to-day for a more experienced rider, but me? I'm feeling darned proud of myself.