Softube makes a plugin called Acoustic Feedback for just such a thing, although I cannot imagine paying $99.00 for a plugin just to get feedback, but who knows, perhaps someday I'll see the need, until then I'll just use one of the few real amps I do have just for that purpose
Just hold your guitar up against a monitor and it works, I know this because from the doom project I used a shitload of feedback and this is how I managed to get mine
Guitar against the monitor usually works, although I must admit to having used a combination of E-Bow, reversing audio tracks, and crossfading to mimic it.