Generated quantites as a mechanism to define model-specific proposals

It just struck me that we might be able to use generated quantities to define proposals other than NUTS. We would need to adapt Stan (once) to do this and figure out how to get it to work but it would dramatically lower the bar in terms of people wanting to define alternative sampling schemes (eg something that exploits the model structure via NUTS in Gibbs or similar) and, once we had the infrastructure, would mean that trying out new proposals would require no changes to Stan’s code. Before getting someone to try this out, I’m curious if this is viewed as insane and/or if someone tried this before. Thoughts?