I took Delmar's suggestion and made a couple of saws from Japanese-style pull saws. The first was a 10.5" replacement blade by Tajima. Very thin metal, carved up nicely. So thin that I added some Basswood on either side of the handle for better grip. Ended up with an 8" blade and weight of 1.3 oz that buzzed through my test 4×4 in 44 seconds.

Small Japanese blade with Basswood grips before assembly

Finished small Japanese saw
Also tried grinding a pruning saw blade to a narrower width instead of putting in holes and slots. to lighten it. The Basswood grips worked so well that I added them to it as well. 11.5" blade, 3.7 oz, 45 seconds on the test 4×4. Not quite as light as the lightest saw with holes and slots, but immensely easier to fabricate:

Narrow pruning saw with Basswood grips before assembly

Narrow pruning saw with epoxied grips
Then I remembered that I had a 20 inch "Sabre Tooth" pull saw in my tool chest and took off the blade and modified it. Ended up with an 11.5" blade, 2.4 oz, 30 seconds on the test 4×4:

Next I obtained an 18" Corona pruning saw and started working on it. This will be a big saw when completed. No final weight of test cut yet.

18" Corona blade marked up for grinding
Finally, I finished the 24" bow saw. A bit of a disappointment. Was shooting for 8 oz, but it weighs 9.9 oz, mostly due to the 1.6 oz handle butt and 1.1 oz tensioning bolt. Was expecting great speed on the 4×4, but the best I could do was 45 seconds, the same as much lighter, simpler and smaller modified blades. Ed Biermann's 24" buck saw is lighter and has more usable blade length, plus a deeper throat for bigger logs.

Will report on the 18" Corona and 12" bow saw when finished.