Just for the record, there are three ways to run flash on top of jack. First is your approach mentioned above.

Second is via libasound’s jack client as mentioned in http://subversion.ffado.org/wiki/ALSAtoFFADO

Third is simply via Pulseaudio’s module-jack-sink, so you can redirect all your consumer applications (vlc, mplayer, skype, flash, etc.) to jackd. This is probably the most elegant solution, as it allows you to switch your *running* streams from your internal (or USB, bluetooth, whatever) device to your firewire device.

Cheerio