I will have to try it to determine it's effectiveness, But it makes sense
https://youtu.be/Uginr22r0zs
Yes/no.
I remember seeing this before as emergency insulation for an under performing sleeping bag for ground pounders.
Will it work, yes. But like a lot of things, everything has to be just right. If it's wet, you will have trouble finding the dry fluffy leaf litter you need. Same for a southern area, you'll have more pine needles than leaves. And in a cold weather situation, I'm not sure I'd trust that not to settle. Convection - Gonna lose a lot of heat from the side not being protected.
Possible upgrades?
-Put the bags IN the hammock with you. You sink into them. Might help.
-4 bags, 2x2. String hammock low along the 'seam' between them.
Alternatives:
Homemade under-quilts
-Cheap 'costco' type throw. plenty of plans online.
-Extra sleeping bag you dont use. String up as an UQ.
-Blue Camping mat (Wouldn't recommend for very cold), but will work in moderate weather. you WILL know where the edges are :zomg:
-Emergency (Mylar) blanket - Can't remember the decision on if you retained more heat due to reflection, or lost more due to convection/conduction in this type use though...
-Budget under quilts can be purchased VERY cheap. Amazon has some currently at $35.
-Wait until Spring and everyone is upgrading and selling old gear...
tl;dr - Plausible, useful knowledge, but applications and effectiveness may vary...