Okay, my initial revulsion for ABI has receded. All things considered, my options aren't that bad. I just had to change my perspective from "huge downgrade from static linkage" to "huge upgrade from a message channel".

Just like a web API, I have to draw a continuous line through the program that separates specific concerns of interest that must fall on one side or another, and which can only cross through things with specific properties.

There are several crates shipping a number of different binary-compatible types, even generic types. Not everything can cross, sure, but maybe not everything should cross either. Maybe a DLL should receive an opaque handle for certain things, such as interpreter internal code representations. Maybe having these separated is important enough to justify having a translation layer.

I'm sure there's much woe ahead, but I'm learning to stop worrying and love the ABI.