I have no personal experience with 650c wheels, but I'm not sure I understand your preference for 26-inch wheels. I reseached them a bit wheh you asked about upgrading your Surly's wheels a few weeks back, and if I recall, you can't go particularly narrow with them. I also wonder how much coin you'd have to drop to get a pair that's relatively light.
Admittedly I'm partial to my relatively light set of custom wheels. If I were going to the trouble of building a custom steel bike, I wouldn't scrimp on the wheels if I could afford not to, and I certainly wouldn't go with something heavy. Your builder may end up having an opinion about 700c versus 650c wheels based on their design.
Out of curiosity, why the preference for 28s? Not that there's anything wrong with them, I'm just wondering about your reasoning.
Live with intention. Walk to the edge. Listen hard. Practice wellness. Play with abandon. Laugh. Choose with no regret. Continue to learn. Appreciate your friends. Do what you love. Live as if this is all there is.
--Mary Anne Radmacher