I assume that LR only uses the actual file name and any EXIF data. If iPhoto is not adding info to the EXIF, it simply won't be seen. I am also assuming that the LR importer is not actually designed to 'read' the database possibly created by iPhoto/Photos. Two "assumptions" is probably twice as many allowed!
I have an app that obviously came with PS Elements that may do what you need. It's called Elements Organizer and has several methods of sorting images including "Events". Unfortunately, Elements is not free, although not expensive (in the Adobe sense, anyway), and older versions can probably be found on the web for much lower prices. I'm not sure you can install Elements in a trial mode, either. Even though there is an "Import" function, it could require lots of manual data entry, also.
I've never actually used this app, so I have no real knowledge of its operation or capabilities.
BTW, Elements is mainly a pixel editor rather than like LR or even iPhoto/Photos. It was originally created to develop the interface for Photoshop
* and can use many 'actions' created for PS because the code is there, but not all the interface dialogs.
* Probably an urban legend, no source that I know of.