I don't like to see forum posts that end with users placing support calls - and then no further posts, so here's a quick update on my issue:
Veeam Support did suggest using the Powershell method of importing the .vbk (my case no. #02220662).
Initially through a backported hotfix for 9.5 U1 - which failed to work, and then following the U2 upgrade - which did work.
Unfortunately that's just uncovered other issues, as the .vbk has been imported as encrypted (despite having no encryption applied in any settings), and no method of decrypting so far!
Looks like it's going to be a busy week!
Regardless, for other users reading this - there is a simple solution for importing backup files from DD, through Veeam Support.