a symlink in /Volumes/External that points to ~/Library/CloudStorage/OneDrive
It's been a while since I've had sysadmin responsibilities, but isn't this backwards?
That is, if you want the files to be stored on the external drive (i.e. ~/Library/CloudStorage/OneDrive should be 'pointing to' as in 'providing access to' /Volumes/External )
A lot of us don't have the option of switching to iCloud because of all of the SaaS products that claim to "integrate" with Office365 (and thus each other) by way of dumping their files into OneDrive. And because we are using Mac laptops along with Windows networks and workstations. The business world (beyond freelancers and small media companies) is not going to adopt iCloud. (And Apple discontinued its Server offering last year, so it knows that.)
This is software that is provided for both personal and enterprise purposes; and it's a pretty fundamental issue with the usability of the Office suite on Mac OS. Pressure needs to be kept on MS to fix it (including as a proxy for Apple). Likewise, Dropbox and Google to the extent that inadequacies in the File Provider Extensions are ultimately to blame.