Got the ReScript in Zed working! This is just stitching together the GitHub - rescript-lang/tree-sitter-rescript: ReScript parser for Tree-Sitter for grammar and GitHub - rescript-lang/rescript-vscode: Official VSCode plugin for ReScript for the LSP.
PR to zed extensions is pending Add ReScript extension by KidkArolis · Pull Request #572 · zed-industries/extensions · GitHub review and merge, after that it should be possible to find the ReScript extension directly in Zed 
And here’s the source if you want to try and compile it locally: GitHub - humaans/rescript-zed: ReScript support for Zed editor
This extension, in particular the LSP does not seem to be working very smoothly yet, it crashes with fs.writeFileSync(undefined) error somewhere in the cli.js quite easily, so will have to continue debugging. But at least this gives you the syntax highlighting!