> In the attached circuit I've overlaid a standard buck-mode converter
> (SW2, CR2, L1), and a boost mode converter (SW1, CR1, L1) that shares
> the same inductor.
>
> Is there some reason that this doesn't work as a boost/buck variable
> voltage regulator?

There are a number of IC circuits that work like this.

For example see http://www.linear.com/pc/categoryProducts.jsp for LTs range 
of buck-boost chips. 

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