I haven't learned how to synthesize with ZASFX yet and I don't know how to synthesize Clavichords properly so any help would be very helpful.
For some reason I can't find a free Clavichord synth (there was one called Clavisynth but it doesn't emulate the sound even remotely even though it claims to)
Overall in regards of ZASFX I'm blown away by what I see, in particular the scaling capabilities. It seems to be no less powerful then Absynth as there's apparently a possibility of drawing an oscillator shape, but I haven't tried it yet...

I found a really good piano preset in the library, but it still lacks some clavichord. If I'll ever succeed myself in making one, I'll post it up.