I've not yet found the precise location of each project, but I have found that it's stored in the photo library. So, to perform this transfer, I just zip up the library and transfer it over. Doing this I can also have multiple sets of projects to test with, and if I need to, I can revert to an older/primary/specific one at any time.
I would still like to ...
Find the Imovie Library
In finder navigate to the Imovie Library file. It should be in this directory Users/NAME/Movies/iMovie Library. Right click the and select Show Package Contents.
Find the right event
There you'll find a directory for each event. Each event directory contains a folder called original media. Those files can be dragged into Photos.