Configurations for Objects Created from Meta Data Card Toolbar

You could use the Object Creator module and have custom create commands for these scenarios instead of creating the assignments from the metadata card. With Object Creator you can populate default values for the new objects and automatically link them to the selected source object (Project in this case).

I would also consider doing the references between the objects the other way around: the Assignment should refer to the Project in its metadata. If you need to have a list of assignments on the Project metadata card, you can use the Managed Properties module to automatically maintain this list.

Thanks, I'll check it out.

Found an easier way to do this and wanted to share. I created a new Property (Project Tasks) as a Multi Select Lookup and filtered the Object Type's Class to be Project Task. Now clicking the create new object button it defaults the class to Project Task because it has to be. 

That's a nice solution!