As promised yesterday, here is the video tutorial showing how to connect the demodulator output of a GNU Radio receiver to the input of the multimon software modem.
Watch video on YouTube.
As you can see the procedure is really easy and it’s a good example of the awesomeness of GNU Radio and Linux 
I think this trick should also work with most digi-mode applications that can read their input from a file. Later I will try fldigi and maybe wxtoimg. Until then go ahead and try it for yourself and let me know about your results!