Thanks, Joe and Jim, for your feedback on my inquery. In the end, I opted for the 20-35mm as both of you suggested. I'm not sure what the going rate is, but I was personally content to pay under $400. As desired, it gives me the compact walk-around lens that I needed for the ist-D, with the built-in flash compatibility that the 16-45mm (which I'll certainly be keeping) lacks. I think it will complement the 50mm nicely.
While the 20-35mm doesn't get me any wider with the ist-D, it'll be my widest lens on the MZ-S. So, it also gives me incentive this summer to dust off my only 35mm body left (inactive for over a year) and see if that slide film in the refrigerator is still any good. Thanks again. - Jerome