I like the idea. I have recently been trying a JRB underquilt and have been frustrated by air gaps and cold air blowing in with light wind. Your design would save quite a bit of weight without the extra nylon, baffling, suspension, and loops and snaps that typically accompany an underquilt.
Furthermore, just as "too much" padding on the ground does not make you hot (because hot air rises...), I don't think that "too thick" of an underquilt would make you too warm if used in higher temperatures.
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