It took me a while to find an affordable 28 hole rear track hub, but yesterday I won one on Ebay Germany.
Vintage Shimano Dura Ace high flange. It was a complete rear wheel, but to save some money for postage, I asked the seller to send me the hub and sprocket only. He can keep rim and tubular in exchange for the effort.
This rear hub will be laced up with a new 26" Mavic GL330 tubular rim. The Giant road bike with horizontal dropouts will be converted to a track bike with 26" wheels. At least I'll give it a try.