ElasticSearch boosting score with external system

I would like to integrate ElasticSearch with external system, over http/rest, for boosting score. I'm not ElasticSearch developer and I don't have too much experience with ElasticSearch.

I can use Native Script for that, but there is a problem with performance. Because for each document I need to call external system. What I would like to see is some kind of batch processing or post-processing for results.

I don't want to store information from external system in ElasticSearch, because they could change in time.

Could you please advise me, how to do that?

Also I didn't find much information (documentation) about custom native scripts or plugins. Just project on GitHub.