Thanks for the post - and the workaround.
I ended up creating two reference attributes - one to each of the Client and Event objects, and then creating shortcuts in the Memo object to the attributes of the Client and Event objects which I was interested in showing on the main form of the Memo.
A problem I encountered was when I tried to implement a custom button on the Event's main form to create a Memo for that Event, the process triggered by the button didn't 'see' the Event or Client objects which had been in context moments earlier.
So I had to create a new SystemSettings attribute to store references to both the Client and the Event objects. Then my CreateMemo process could happily go create a memo for the Event and could cross reference it to the Client too.
Not a tidy way of doing it, and not what I expected would be needed, but it works.
Maybe support can tell us if a shortcut to a reference attribute's reference attribute's attribute should work ....... I think I got that right 🙂