Here I attached a proof of concept patch for the same.
Any suggestions/comments on this proposed approach?
If I understand well to your proposal, the major benefit is in impossibility to enter pg_hba keywords - so you don't need to analyse if parameter is keyword or not? It has sense, although It can be hard to do image about pg_hba conf from these partial views.
There can be other way - theoretically we can have a function pg_hba_debug with similar API like pg_hba_conf. The result will be a content of pg_hba.conf with information about result of any rule.