You can get off-shelf DC/DC modules that will do this relatively easily, but I would be concerned that a phantom supply couldn't provide enough current to do this reliably. IIRC a mic requires around 7mA from the phantom supply which isn't exactly a lot of current unless it's a mains voltage and you are standing on a wet floor....
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BSS (who made a couple of types of DIs) used a CMOS 555 (7555) driving a transistor driving a little pulse transformer (amazingly, available at RS components) to generate isolated power. Various condenser mics (AKG etc) have phantom driven DC/DC converters. The maximum available power from a single phantom powered mic line is around 150 milliwatts. You will run into the contradicting op amp requirements of low noise, fast slew and low power (choose any two ).
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You get +48V on two 6.81k resistors so by 10mA (5mA/side) you are down to <14V. If you are not using much Isupply you could make +15V in the conventional way and use one of those flying cap doublers to make -15V. There are a lot of sites with ideas for the classic Schoeps transformerless mic circuit, it's a good start.
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