In case it was misunderstood, the entire piece in question would sit within the splined cranks, except for the inside shoulder that I mentioned in my second post. Assuming such a set up is possible, why would the crank arms be moved so far apart (up to 3", by MikeW's estimate)?

Of course, BCRider is right about the practicality and desirability of such an adapter.

However, I will say that this idea came about when thinking about the type of bottom bracket to use in an upcoming build and dreaming of far-off touring: wrestling with the serviceability of cup-and-cone bottom brackets, the availability of cup-and-cone vs cartridge bb's in such places, the disposable nature and increasing prevalence of cartridge units, semi-cartridge bearings like Phil Woods and the matching axles, required tools, etc.