Hi, yes I'm quite interested in getting involved though my time is limited for hobby projects right now. After investigating briefly when I originally answered your question, I suspected you may need to modify core. It seems like a good idea to look into whether the Admin plugin can be extended, since this will already contain the API operations. You could write endpoint templates with Twig I guess.
You might find it useful to look at the devtools plugin code for tips on porting some Grav operations for another client environment (command line). Devtools itself would in fact be greatly strengthened by the inclusion of RPC methods, in line with drush for Drupal.
How are you planning to work on this if you find a feasible path? Will you be developing in the open and giving back to the project? I suggest touching base with project lead @rhuk about your interface before going too far down that track, though I am pretty sure he'd be thrilled to have it included.
Cheers