Trying to solve this has been an odyssey, thanks in part to Adobe's new community platform breaking old links. Most of what I found is in this thread.
So for some stupid and arbitrary reason, document bleeds can't be set via script. They can be set with a plugin, but thanks for breaking that link, Adobe! I can't find an archived or cached version of that thread anywhere.
The source for a Mac plugin is here but I have no idea how to compile it, and the developer provides no documentation on how to do that. I got as far as downloading the Illustrator SDK and pulling the plugin repository into Xcode 11.3, setting the target to MacOS 10.13 as specified in the SDK documentation, but I can't get it to build, it can't find IllustratorSDKDebug.pch (and neither can I).
I'm lost, and beyond annoyed that something that should be as simple as declaring a four-element array in JS has turned into this. Any help with this would be hugely appreciated.