Da! This is supported. Please see my thread on the current
Status, which includes a link to my fork/repo. I now have ScriptCraftJS working with v1.12.1 on my systems. I'm working on the documentation and would appreciate some ALPHA feedback on non-production systems. I really do not want to expose this work-in-progress as a Beta, as though it's close to release. At this point I don't know what I don't know, so I'm not sure how close it is for a broader evaluation. I have not yet had feedback from Walter about this fork or how soon we can port it here to get it closer to production.
But YES, we are moving forward!
I have completed and closed some open issues, and have provided feedback on others. For now, the very next v3+ release will not include significant fixes or enhancements - but there are some in there (see Release Notes in my repo). The goal for this next release is to publish a good version that we can all use with the current Minecraft v1.12.1 + Spigot + Java 8. In the following updates we'll look at enhancements. I will also strive to publish near-immediate updates for new Minecraft updates so that we can continue to use this fine software with no interruptions.