If you want to get an array with the keys bonded with an Action mapping using C++ you would need a few lines of code. Trying to achieve the same result in Blueprints would result in, well, tears. Could you please make PlayerInput acessible via Blueprints? It would be a nice feature in 4.15 
And for those of you wondering about the code to retrieve the keys set in the Action Mapping, call GetKeysForAction on the PlayerInput.