Skimming the merged eRFC and discussion thread I don't see any/much discussion of this, presumably because it's not important for the #[async] -> impl Future case that motivated this particular eRFC. There has been discussion of this in the past on the more fully-fledged stackless coroutine RFC and Post-RFC threads that preceded the current implementation.
I'm actually really curious if it would be possible to use this to implement an #[async] -> impl Sink and even #[async] -> impl Stream + Sink variants of the #[async] macro, these would be really useful for me to help implement async IO adapters.