Jeff,
You can change the location where modern apps are installed with this regkey:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Appx
PackageRepositoryRoot
PackageRoot
By default these are set to:
C:\ProgramData\Microsoft\Windows\AppRepository
C:\Program Files\WindowsApps
I think only the Repository one matters. The paths are locked down with system access only, so you'll have to use the advanced tab to reset the permissions to view the files. Setting these regkeys to a SD card is not recommended for performance and resilience reasons. I understand what you're trying to do here, but it may not work out that well. I don't know of a way to redirect local storage for just one app, its all or none.
There are probably other approaches here, but this is the only one I could think of.