The exact nature of the loopback device is confusing to say the least. I've always used it with just one soundcard - either with Jack running on a firewire device or on my onboard soundcard, and all alsa audio getting routed to / through loopback.
When you take an input from your AF12 and connect it through jack to the loopback device, that makes it available to alsa applications as an audio source - ie, you could use a mic into your firewire device to talk on Skype, for instance (I've done this). To loop that audio back to the loopback, then, would take another piece of software (something for alsa monitoring), but that would only succeed in getting the audio back into the Jack system and no closer to your MoBO soundcard.
To do what you want to do, (if it's even possible, I'm not sure), I believe you'd need another instance of zita-aj to pipe audio from jack to your alsa soundcard, seperate from the one that pipes audio from jack to the loopback device. I'm not at all sure of this though. But I believe that since you want to take Jack audio and route it to a real Alsa soundcard, you would use zita-j2a in a similar way to how you use it to get audio to the loopback device, only pointing it to the real soundcard instead. The .asoundrc just tells alsa-aware applications to use the loopback device as their default soundcard, enabling these applications to actually find your jack audio ins and outs.
This explanation comes with no warranty expressed or implied. Use at your own risk.