I also did something similar to postgREST for MSSQL in java, not in haskell because of the same driver issues you mentioned. I managed to implement jwt auth pgREST style taking advantage of MSSQL user impersonation though I did jumped into some limitations, for example the lack of enviroment variables in transaction scope in MSSQL forces you to have to declare one db user for an app user.

I have little experience in Haskell despite that it was easy for me to understand the gist of what postgREST does and port it to java.