@applee so yeah, my swapping-out of fstab isn't really bringing anything to the dynamic-mounting party. One can do it just as easily with:
sudo mount -t cifs -o username=<username>,password=<password>,rw,uid=32011,gid=32011,sec=ntlmssp //<server>/<share> <mount path>
(With appropriate replacements for <these fields>). The uid and gid numbers are those for the phablet user that UBPorts gave, as shown with the "id" command).
However I still think that changing fstab is better in some ways, since the shares should mount automatically, that is, if UBPorts wasn't clobbering the fstab. It seems like it should be possible to share that function with UBPorts' remappings. Of course it would fail to mount automatically if the network or share is not available, but that seems to be ok on other distros, it just doesn't mount, and you mount it manually later. It also lets the admin set it up so a user can mount it without having the technicals. But again that's a bit different on UBPorts, since it isn't really multi-user.
It seems like there are other things in /etc/ that are generally necessary to modify, /etc/ssh/ssh_config comes to mind. Isn't that where any systemwide config would be? I'm not sure how that could be unmodifyable.
Maybe I'm just expecting too much Linux out of it...