What I do at present is to add two fields to the observation (example). One as others have said, “Interaction->Visited Flower of” and another, “Associated observation”. The second field has a link to the observation.
However, to search for these observations is not yet straightforward, so the project approach that Tony suggested might be a better short-term solution. There is also a feature request to add interactions to species pages with some existing functionality, but not based internally on iNat observations - please vote for it if you would like to see this happen!