> Schottky, resistors and cap!

Another thought -

The circuit is split in two, say micro and other, and each side is
powered via a diode. The normal power is applied at the point
where the two anodes connect, with the back-up supply across
just the micro

              power
                |
                |
  micro ----|<--+-->|------ other
    |
    = backup cap
    |

wbr
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