I've recently seen the ads for NI's Spektral Delay, and I'm wondering what it might do for a piano. I think it's intended for more serious sound warping, but I'm thinking along the lines of using it try to simulate sympathetic resonance. The description on the NI site is remarkably general--I'm not sure if you can control its effects with velocity, for example--and I don't want to spend an hour or two downloading and installing the demo and finding out I can't.
Anyone played with this? Looks interesting. Uses FFT in realtime to let you control both stereo channels, delaying\changing the amplitude of harmonics on "up to 160 separately modifiable frequency bands" in each channel. Hmm...
It's a fantastic sound warping tool. Well worth experimenting with on all sorts of sounds. If you have any interest in that sort of thing, you'll love it. If you're only considering it for the purpose of sympathetic string resonance for a piano, I'd say it's not the right tool for what you are thinking of doing, and you will be frustrated by it (to answer your specific question, I don't believe you can make it velocity sensitive, but I'll double check on that and post again if that is incorrect).
Having said that, download the demo and have a play. If you have any interest in sound design at all, you'll find something in there that will bring a big smile to your face I'm sure
Spektral Delay could certainly do this, but I agree that it is not at all the primary purpose, and the UI is not ideal for it by any means.
GigaPulse is the tool for what you want to do, in terms of what is commercially available. You can set up any convolver to produce a basic resonance, but doing it in GigaPulse gives you more control over what part of the tessitura you want feeding into what aspect of resonance. You can either program very generically overall, or keep drilling down and separating things out.
If you want to get very directly to the point, go record some harp/body resonances by striking the piano bridge with an impulse hammer (or dropping a big ball bearing, 3/4 to 1" in diameter, then catching it on the first bounce). Do pedal downs and pedal ups. Then drop these into GigaPulse, make the assignments, and start pushing the samples through them. You will have a genuine body resonance.
The single thing I'd mention is that exciting a piano, short of purchasing a very expensive calibrated impulse hammer, is an inexact science. So, you'd want to experiment liberally with EQ and balance of raw tone to impulse, in order to achieve a plausible, realistic resonance. But the tools are there, to both capture and apply the real McCoy.