To my knowledge most guitar processors can simulate feedback. There's no mystery to it since the process is quite simple. All they basically do is run a sine wave with slow attack and a touch of vibrato through distortion. In some cases it can be set to trigger automatically but on most Boss and Roland processors I've used I could also trigger feedback with the pedal.

I used to do this with my synths too. Set up a "feedback" patch, layer it with some nice electric guitar sample (or fake it best I could with analog synths) and just run the whole thing through distortion. THe "feedback" patch should be at least a couple of octaves higher than the guitar sound. may be even 3 or 4 octaves just to make it really really hurt your ears. Plenty of vibrato on it too and a slow swelling amp envelope attack settings.

cheers,