Yep! There are probably issues with this setup, but I wrote a small plugin to allow them to work in the same repo, sharing the same express instance: https://github.com/coxmi/astro-payload-plugin
Because of various ES module/CJS issues I had to bundle payload with esbuild and change the extension to cjs. The config wasn't being properly found/parsed by their webpack/ts-node setup, and because payload loads its own config in dev mode, it's all a bit complicated. This was the easiest fix I could find.