It looks like this already may be possible, unless I'm misunderstanding the exact use case:
Generate a package-lock.json file without installing dependencies in node_modules: npm i --package-lock-only
I was only using pnpm for demo projects to save disk space because I needed a package-lock.json on the CI servers, but it looks like I can just generate it via npm while keeping the node_modules symlinked to a central store. I think this means I can now use pnpm for work projects too.