Seem to have an answer to my own question, or at least a work-around (and it only took a couple hours of trial and error): Chose "L" as the media size (photo 3.5"x5.0") in the device driver. Tell the word processor that you want a 3.5"x5.0" page size. Skew the margins 0.25" to the right. Put the 3.5"x5.5" blank US Postcard into tray #1 with the face down and the "stamp" to the left and the short side first into the printer (i.e., the left end of the Postcard). Print the address. Turn the Postcard over and repeat to print the message. Noting that dual sided printing will not function with and card stock. Bottom line: the answer is to lie to the software, twice. Then go ahead and print. If you have the address and message in the same doc file, be careful to print only one page at a time and be sure to get page 1 and page 2 in the correct order, or you will get the message on top of the "stamp".
Convoluted, ain't it?