Hi,
With the “old” JSLink setup option on SP 2013, DFFS was loaded successfully into the “NewDocSet.aspx” page. Unfortunately the new DFFS installer / loader is not able to write to this page because it is a application page in “/_layouts”. Unfortunately I didn’t think of this when designing the new installer / loader.
The only method I know if that actually can be used is the JSLink approach. I’ll try to find time to write a new “installer” for the “/_layouts/15/NewDocSet.aspx” that utilizes the JSLink loading method, but this will not work with the new “experience” – so if this rolls out to the “/_layouts/15/NewDocSet.aspx” page the JSLink method will fail to load the script.
Sorry for the inconvenience,
Alexander