Actually, it isn’t quite that bad.
While you can’t select the entire field and type in a date, you can type in the individual components of the date.
For example, to do this in the Inspector:
Double-click on the custom date field to launch the entry pane.
Double-click the month to enter it.
After you’ve entered the month, the cursor should jump to the day.
And when you’ve entered the day, it will jump to the year.
Or you can tab or shift-tab to the date components.
(Only reason I thought to try this was because Quicken works the same way. 
)
Unfortunately, there doesn’t seem to be a good way to enter dates in batch, for multiple documents. 
Best,
Jim