I ran sudo spctl --master-disable and the problem doesn't happen anymore. I can successfully image a USB stick. Of course, this isn't an acceptable work around for users, but I think it conclusively shows it's a signing issue of some sort.

@puiterwijk does the mac builds and would be able to look at this.

This turned out to be a known bug in Mac OS 10.13, with a documented workaround/fix at https://developer.apple.com/documentation/security/notarizing_your_app_before_distribution/resolving_common_notarization_issues#3087725 .
I intend to make a non-stapled build and distribute that when I get time to handle this.

Any update on this?

Well, near as I can tell, Apple did kindof a craptastic thing with this bug by basically saying they're not going to fix it in 10.13 and users should just upgrade to 10.14. And now that 10.15 is out, and don't officially (i.e. they don't actually tell us one way or another) say what the support level is for n+2, I think it's reasonable to just dispense with this bug and any other bugs related to 10.13. (And I say that as a user who for like a two decades considered macOS my primary OS and now Fedora is my primary OS). Since Apple's way of "fixing" this bug is to say "upgrade" it's totally sane to tell our users the same thing, as crap as that is it's at least clarity, rather than ambiguity. shrug