An update. The peach upside down cake turned out well, but not great. Which is to say: it as delicious, but soggy. Very, very soggy (particularly around the middle bits). But it was so delicious (and, though I hate to say it, low calorie. I'm not a fan of diet food, but I do like it when foods I like are also foods I can eat a lot of, yk? Call it gluttony, call it hedonism, just make sure I am well fed!) that I was willing to experiment a bit.
So I made the cake again tonight. It was a bold move because we were having friends over for dinner, and what if my cake was a soggy mess again? But I faced the peril. This time I made a few changes. First of all I used plums rather than peaches. I don't think this decreased the soggy potential, though, because these plums got a little too much rain during their last few days on tree, which is why I was baking with them in the first place. Next I decreased the amount of liquid in the batter and the glaze, and for my final tweak I baked the cake longer (I figured there was little risk of it drying out, given how much juice the plums were producing in the bowl where I sugared them).
And voila! The cake is delicious. Delicious, I tell you! I even left the butter out this time (this probably helped the cake bake up better- if you do decide to add butter to the glaze you might consider cutting the batter liquid a bit more), accidentally. No loss there, as far as I can tell.
I have a new go-to fruit cake recipe!