This is still not quite ideal with regards to the original question, but better than a solution affecting all Terminals: from this thread I discovered that from the desktop (i.e. if I first switch to an empty workspace, or otherwise give the desktop the focus somehow) I can Win+Alt+T to open a Terminal with cwd at the Desktop.
This tidbit of knowledge has the additional advantage that, if I were doing this to then cd
more easily into a folder on the desktop (to run a newly downloaded app, for example), I can instead just open the folder and then hit Win+Alt+T.