The fact that Neoair pads are competitive on a warmth per unit weight basis with pads using other kinds of insulation (foam or fiber) demonstrates that radiant barriers can be effective for sleeping pads. But, for your application, I see several things that might be worth considering.
Sleeping pads containing foam, down, or synthetic fiber insulation are good at impeding convective heat loss to the ground because they contain a very large number of very small air spaces. They are good at protecting against conductive transmission of heat because the solid parts of the insulation (the polyester fibers, down plumules, or foam cell walls) create a "tortuous path". If we imagine heat as a packet of energy travelling through the solid material of the pad, it would have to travel hundreds of meters just to make it through a 2" thick down-filled pad because the plumules are long and the contacts between them are few.
Neoair pads and Double Bubble Ecofoil don't have any of those qualities. The air spaces are large and few in number and the solid path across the material is relatively direct. But they have multilayer radiant barrier films (two radiant barrier layers in the Ecofoil and more than two in the Neoair).
Imagine that a square foot of the surface of a sleeping hiker is radiating 64 watts of heat (this is radiation only). If you put something opaque, like carbon paper, very close to the surface of that hiker (but not touching it), then the carbon paper will recieve almost all of that 64 watts of radiant heat, and it will radiate half of that back to the hiker (32 watts), and half away from the hiker, from the other side of the paper. By adding one opaque radiant barrier layer, you have effectively cut radiant heat loss in half (reduced it by 32 watts). If you add another opaque layer (not touching the first), you cut it in half again, but the real reduction this time is not 32 watts. It's 16 watts. A third layer reduces the radiant heat loss by half again, but this time that's only 8 watts. By the time you have six layers, adding another opaque layer cuts the heat loss by less than one watt. This is a general rule, and it remains true for your application despite complications like aluminized surfaces and thermal bridging between layers.
The Ecofoil probably doesn't actually have an R-value of 3 (the R-values of building insulation is usually grossly overstated). But even if it does, it would only achieve that when it is the only radiant barrier layer. Underneath another radiant barrier (like a Neoair), it would be no better than bubble wrap, which has an R-value of about 1.
So, using a radiant insulator over a conductive/convective insulator (like a NeoAir over a foam pad) works great, because each component takes care of the kind of heat transmission that the other component misses. The other way around (a down air mattress over Ecofoil, say) also works great. But using two multilayer radiant barriers together, like a NeoAir over Ecofoil, will not achieve the improvement in warmth that you might expect given their independent R-values.
I think some of this might just be a restatement of the good points that Eugene already made.