Is it possible for a Custom Visual to edit data in a SharePoint List from Report Server

Data writeback is a weak spot in Power BI.  Currently you are limited to PowerApps or PowerAutomate visuals, or to R/Pyhton scripts. Not sure if the Javascript for custom visuals supports writing to Sharepoint.

Also consider that even if you succeed, the customer UX will still be broken as the data changes will not be reflected in the dataset immediately.

Thanks for the response!  So, it's at least possible to create one with R?  Can I use R only, or would I have to use something like javascript along with it?

I _think_ so.  It's on my list of things to test. Why don't you give it a go as well and we'll see how far we can push that rope.