> As a more general comment, using a GC language as the FFI target from a GC language is begging for difficult-if-not-impossible-to-debug crashes down the line.

Not true!

What you do is that you keep a registry for objects passed from the host vm to the foreign vm in which you register objects thus transferred. And you use a similar mechanism for objects passed from the foreign vm to the host vm. In CPython, you simply increment the refcount to prevent Python from collecting them prematurely.

This is how Java does it (via JNI) and how many gc:ed runtimes interact with other gc:ed runtimes. This is how you do it in Rust too since Rust can't tell how long an object passed to a foreign vm is supposed to last.