Dan--I tried three summer cuttings with a close approximation of your method, but may have missed something essential.
These were partially-lignified cuttings of LSU Purple, Conadria, and Violette de Bordeaux. None seemed right on the verge of a growth flush, but have been growing steadily and I wanted to get started. I trimmed the leaves as per your description and placed them in a wide-mouth bottle (no Ziploc) with a couple of inches of rainwater; however, the cuttings were somewhat bowed, leaving 3-4 inches of the lower end submerged. Water was shaken and/or replaced daily; any fallen petioles were removed promptly.
The LSUP cutting (15" long but fairly thin, less than a pencil-width) quickly started going soft on the wet part, so I snipped off the bad section and put it back in the water. The "new" bottom went soft as well, and then the top shriveled, so I figured it was a loss and chucked it.
The other two seem like they're probably still okay, although now the wet petioles have gone soft and disintegrated, rather than separating cleanly at the normal abscission point. I hosed them off today, which removed most of the slimy tissue, and changed the water again (this time sprayed from the hose, so it should be very well-oxygenated). They're back in the water again, but this time I tipped them so only an inch of each is submerged. Neither of them has been in direct sun since day one.
For the remaining two, are they basically on track? Did I miss something important? Any clarification would be appreciated--thanks.