Jay Tee 4303
That's a variant of my suggestion, it should work well, and yes, the key is bringing the wet signal back into it's own channel input, not the FX return.

The light just went on! :) This was the missing piece - bringing it back into another channel. The 1200f is really powerful, and I have 12 outs as well as 12 ins so plenty of room to do this. I can then blend the 100% wet return in the DSP mixer and send to the headphone outs - no need for outboard mixing gear; at least not for one signer doing overdubs.
 
thanks! :)

FWIW it is possible to add 'comfort' reverb/echo 'in the box' without suffering latency problems. The trick is to send the singer a direct monitor feed from the audio interface and not from the recording track.
 
Set up a pre-fade send from the recording track to a bus effect with the reverb/echo on it. Make sure the effect is 100% wet & 0% dry and mix it into the monitor feed to the singer.
 
Enable input monitoring on the recording track so that it will send the incoming signal to the reverb bus. Pull down the track fader so that the latent version of the singers vocal is not fed to the monitor.
 
This will record a dry vocal and will be able to subsequently add whatever effect the song needs.
 
The fact that the reverb effect is delayed slightly by the latency of the interface is not relevant - just think of it as a bit of free pre-delay :). Your singer will not notice it as long as your buffer settings are 'normal'. Their own voice is not at all delayed, which is the important thing, because you are using a direct hardware monitor feed
 
Of course, an outboard device is more convenient, but not essential.