It might be interesting to play with biasing the various nodes in the diode string with a DC offset......oh yes, and the base/collector node on any "common" silicon transistor also makes an interesting "soft" diode.
I've seen a few of these but all were for guitar. Other combinations of silicon and germanium will yield different distortion effects. Also try putting the string of diodes into the opamp's feedback loop. Use an opamp that is somewhat tape sounding, like a BurrBrown 604. It will also drive the diodes better than TLOx opamps.