The grey screen thing, it turns out, is just because I didn't run the .swf from the bin/ directory. It runs just fine once I put the output file there!
Thanks for this. Just to confirm https://github.com/Draknek/fcsh-wrap is what you are referring to? When I tried the Flex SDK I had troubles getting it configured and pointing to the correct files.
For what it's worth, I built it with 4.6.0 of the Flex SDK, which Adobe is still hosting: https://fpdownload.adobe.com/pub/flex/sdk/builds/flex4.6/fle...
So yeah, problem solved, I guess?