This seems a bit racey, as in you cannot be sure that the next request is the result of the original redirect, especially if you have a few tabs open and applications making ajax requests. 

What do you think about maybe generating a random flash scope key, and appending it to the redirect URL as a path parameter?

Basically you would take a redirect like:

http://mysite.io/foo?param1=val

and rewrite it to:

http://mysite.io/foo;_flash=$FLASH_KEY?param1=val

You can then use the incoming path parameter to determine the correct flash scope to retrieve. 

I'm not 100% sure if this is nesseary, but it just seems like the current implementation may not be 100% reliable.