I want to use Link Detox Boost only for indexed sites, and so exclude all the links that are not indexed anyway
It sounds like you believe that using Link Detox Boost is only required on *indexed pages*.
The point of BOOST is however to get pages *crawled* no matter if they are indexed or not.
Only when a page is crawled, the Disavow command for this page is processed by Google.
In fact, running BOOST on de-indexed (penalized) pages is often more important,
for making the disavow file work.