After various unsatisfactory attempts to provide a sane implementation of implicit parameters as a library, my conclusion is that this must be a built-in GHC feature.
This is the proposal: Nominal implicit parameters by lortabac · Pull Request #741 · ghc-proposals/ghc-proposals · GitHub
A prototype is available in my GHC fork: Files · nominal-implicit-parameters2 · lortabac / GHC · GitLab. The implementation needs some cleaning but is quite complete in terms of features (only marginal features such as TH and HIE support are missing).
Let me know what you think.