So, with the GNASH_OPTIONS=writelauncher hint from last time, you have got a script which runs gnash. It is still running a remote SWF file, though - you want to modify that file to isolate a test case, and for that it will need to be on the local filesystem.
First, look in the gnash-debug-1.sh script to find the URL for the SWF, and download it. It's the last option passed to gnash.
In order to convince the file that it's still being served from the web server, you now need to swap the '-U' option in the gnash command line for a '-u'. This is enough for gnash 0.8.8 to play the file.
Unfortunately, if you are running current git master, this does not work. You also need to feed the SWF from standard input to gnash - i.e. use '-' as the last option, and redirect the SWF file to the gnash command:
/path/to/gtk-gnash -u http://some/url [more options] '-' < downloaded.swf
If any of this is documented, I'm buggered if I can find it.