search_api_solr with accurate radioactivity sorting in views
As you say there's no API, so you're probably going to have to resort to patching the module.

There are at least two places you'll need to make additions as far as I can tell:


_radioactivity_update_energy()
radioactivity_cron()


I would suggest adding your own hook which gets fired whenever an entity is updated in either of those functions, and perform the search indexing there.

You should have a good think about the values you use for half lives/Solr processing delays if you choose to implement it like this. If your values for those settings aren't sensible you could end making the changes without actually seeing any effect. Also, depending on the amount of traffic the site receives, you could slow your server down quite a bit doing this.