Can I use Dietpi-drive-manager (or another command/function) to mount a remote NAS drive (with dynamic IP, so I would be using a DDNS address) on an RPi3 and/or Odroid xu4, then use this remote NAS as the source for media files for the locally installed Logitech Media Server or another media server ?
yes, fstab can do that…but it can’t map a dhcp address (a proper named one should work)…the one it maps to will have to be a static that is there every time
there are numerous links to show how to do that
https://askubuntu.com/questions/157128/proper-fstab-entry-to-mount-a-samba-share-on-boot
https://wiki.ubuntu.com/MountWindowsSharesPermanently
https://discourse.osmc.tv/t/configuring-fstab-based-samba-share-mounts/38167
Thanks for your reply but I’m not sure the info in your links applies to my use case. What I want to do is:
Put an Odroid XU4 at one house
Mount a remote NAS at another house using a www.ddns.com address as a local drive on the XU4 at the other house
Leave the XU4 at the other house, install Logitech Media Server there, and have it use the remote www.ddns.com NAS drive as data source for XU4 media files
I assume this would be a fairly normal thing to do and so would be possible. Any more help would be appreciated! 
As long as you don’t setup a VPN on the remote NAS, usual network drive mounts like NFS or Samba are not possible, and even with VPN, those protocols are not really made for transfers outside a local network.
Not sure about SFTP mount possibilities, but FTP, since unencrypted, is nothing I would recommend, or only with a VPN connection.
EDIT: sshfs (SFTP) should match best your aims. Here a guide I just found: https://www.debianadmin.com/mount-a-remote-file-system-through-ssh-using-sshfs.html
Implementing this as server and client/mount into DietPi is long planned but still outstanding
Another approach would be Syncthing (or similar), but that means that all data is stored on the XU4 as well, which is most likely not what you want.
Nextcloud plans to integrate some virtual drive feature with their desktop sync client, but this seems to still take a while to be released.
Basically I would have NAS and XU4 on the same location, respectively the same machine and use a web-based media player to play media on any remote location 
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I have Emby running on a device at home and when away from home can view media and play music, not by drive mounting but by port forwarding - you can achieve what you are looking for easily I think?
did some digging SSHFS
In computing, SSHFS (SSH Filesystem) is a filesystem client to mount and interact with directories and files located on a remote server or workstation over a normal ssh connection.[2] The client interacts with the remote file system via the SSH File Transfer Protocol (SFTP),[3] a network protocol providing file access, file transfer, and file management functionality over any reliable data stream that was designed as an extension of the Secure Shell protocol (SSH) version 2.0.
might try this
Or maybe have the local VPN into the network you want to mount and have it mount that way?
Well that is if DietPi and it’s kernel builds support SSHFS
This way…it can connect and mount securely thru SSH…which is a VERY good thing because it’s a secure tunnel
This listing shows a pretty nice way to set it up