Why is the symlink I created inside an application container to allow for external storage not working?

I was trying to get around Vemedio's Instacast limitation on storage location by symlinking (ln -s) the Episodes folder in ~Library/Containers/com.vemedio.osx.Instacast/Data/Documents/Instacast to another location on another drive (which is actually internal in my Mac Pro for what it is worth...). I am wondering if Apple's sandboxing is preventing this to work... Once I open Instacast, none of the previously downloaded episodes are marked as available locally and if I try to redownload one, it seems to download the file in the background but never makes it available... Any idea?