Hello Ben,
The true is that the solution i found it's really easy.
I understand your problem because I have the same type of client, but the trick is create a continue FTP connection with the client.
You just need to create the first connection point and the user never going to note the difference, because they're going to access the folder like a system one.
It's really easy if you put the FTP folder like a Map Drive, or in Mac, like a OS Folder.
About security, In my case I created a FTP user with permissions for only the gallery folder.
After that, the user just need to use the submission for select the gallery that they want.
Here a image of how's "submission" looks like:

(The system language in the sample picture is Spanish)
Regards,
R.v.