I've pushed an update to libffidemo, adding the class DemoFFIAutoThing and tests.
DemoFFIAutoThing sends #autoRelease to its handle when said handle is filled by libffidemo with a proper pointer. To support auto-release, DemoFFIExternalObject implements the following class-side method:
Note the C function signature, which is different from DemoFFILibrary>>apiFree:'s:
DemoFFIAutoThing>>letGo becomes a no-op. Making it send "super letGo", which effectively calls DemoFFILibrary>>apiFree:, crashes the VM.