some synths have special guitar patches that articulate differently according to velocity (like a hammer on if velocity is higher than some threshold)

I've also seen upper bank patches on some synths that do nothing BUT the crazy little sounds you just can't get from a GM guitar patch

If you have a hardware synth, explore the upper banks... you'll probably find some interesting solutions. Older Roland modules have a lot of these patches.

But, it still takes a lot of patience to build a convincing guitar part that way...

(gotta love those Real Tracks!)