From the user perspective there are more interesting aid modalities other than direct donor or core contributions. Pooled/basket funding is as interesting as (un-earmarked) core funding.
Adding this functionality to the standard can easily be solved in the activity standard by simply introducing one additional code list element 'aid modality' having values 'project', 'pooled/basket' , 'core', 'private funding'. No need to have a complicated extension of the organization standard and additional guidelines. It would answer your question if an activity is funded from private funds, core funds, a multi donor initiative or from direct donor contributions.
I.m.o. no need to have a large change to the organization standard and guidelines when we can solve this information requirement with a very simple backward compatible change to the activity standard.