Never posted the resolution to this problem... After checking everything I could think of (and everything you all recommended) under the car with absolutely no success, the path to the ultimate solution dawned on me.
Actually, a totally different problem with the transfer case in my wife's Jeep tipped me off... In short, just because a sound seems to be coming from one location doesn't mean it actually originates there: resonance really was playing with my ears here.
In the original post, I noted that the whine sounded just like a loose belt. Well... you guessed it: it was an under-tensioned V-belt on the air conditioning compressor. To me (and everyone else inside the car) it sounded like it was emanating from below. It wasn't until I asked my wife to drive and I stuck my head up and out through the sunroof that it became apparent that the squeal was from under the hood.
Upon rereading my original post, it seems so obvious; but then hindsight is 20:20!
Thanks for the support. Glad to report that this mouse has been trapped!:p
PS: What a pain in the butt loosening the fastening bolt on the tensioning pulley is! REALLY tight quarters in there, and the bolt abslutely refused to budge. With no other recourse, I eventually bit the bullet and bought the specially designed 10mm hex socket tool (117 589 05 07 00) to generate enough leverage to do the job...
1991 560SEL-Euro (145,000 miles)
1989 420SEL (Retired at 325,000 miles)