I have not seen anyone but Tanzer use this term, and he uses it frequently, both for whites and reds. Here he describes four California chardonnays, scoring each of them 90 or 91 points:
"...Gains weight and a buttery, nutty quality on the long, sappy finish."
"...Dry, cool, dense and brisk, with sappy citrus and fresh herbal flavors."
"...At once generous and sappy, with complex flavors of yellow fruits, apple and spices. Finishes with very good grip."
"...Explosively aromatic nose of sappy yellow plum and smoky, charry, resiny oak."
Here he describes an Australian shiraz, scoring it 95 points:
"Inky ruby. Hugely aromatic, pungent bouquet of dark berries, kirsch, fresh flowers, licorice and minerals. Expansive dark fruit flavors completely suffuse the palate and are brightened by vibrant notes of fresh flowers and graphite. Wonderfully creamy in texture, with no rough edges. Tannins slowly sneak in on the finish, adding structure to this dramatically concentrated, sappy shiraz. The persistence of fruit here is amazing."
So he clearly uses the term in a positive sense. Unfortunately, he does not define this term in his own "Wine Glossary" on his website!