Python is different since most people use the same implementation (I realise the whole FFI situation there is currently undergoing an overhaul to better support

Python is different since most people use the same implementation (I realise the whole FFI situation there is currently undergoing an overhaul to better support PyPy) And https://github.com/ktakashi/r6rs-pffi https://github.com/ktakashi/r6rs-pffi seems to show that such a thing is possible for Scheme as it supports half a dozen different R6RS Schemes, but (I believe?) it needs library authors to target the PFFI interface rather than the implementation-specific FFI interface of a particular Scheme as most do currently. So it seems like that shows that a compatibility layer is possible, at least for many of the major Schemes, and it "just" needs standardisation and adoption.