Also consider TI simple switcher regulator ICs.  There are inverting buck-boost examples that make negative supplies without too much effort.

A simple charge pump can be made from a strong square wave output.  Digital I/O is limited to 5V. A bipolar NE555 timer or a bipolar op-amp can swing an output much higher.  The 555 output can drive at 15V.  The CMOS 555 is weaker.  An inverting CP can then be built from film caps and schottky diodes.  If you need more voltage or current than a 555 can supply, then you might need discrete MOSFETs and a half-bridge gate driver.

Does the datasheet for your driver list anything like a maximum dynamic current? If you are switching many data lines rapidly, that current would be much higher than the device's idle current. +/-12V is also lot of voltage to slew, that limits your data rate. (If you haven't, read up on RS-232.) 

I won't be using an off the shelf driver chip because the waveform I will be driving is a weird one.  The voltage also won't be swinging rail to rail all of the time; that's only used when the baud rate is forced to drop during periods of extreme noise. Either way though source impedance will be 100 ohms so I'm sizing the power supply for a shorted output.

Attaching a circuit to an arc welder? :)

The ltc3260 regulators are limited to 50mA each, you might look at a constant-current loop instead. Check out Low Voltage Differential Signaling (LVDS) on Wikipedia. A single ltc3260 would easily drive multiple data channels.

Yeah, actually. This is for a general purpose laboratory/manufacturing automation system where modbus just didn't fit the bill. Sometimes I need to pass high bandwidth process data to a server, but in other cases I need to send 50 bytes per second to a 17kV "thumper" that uses rapid discharges to make a porous oxide film. 

Put all 24V to work with an H-bridge. Bits go in, 48V differential signal comes out. A comparator at the other end to get the bits back. Reduce the loop impedance until the noise goes away.

There's also optical, like IRDA, for the low speed link.

Just buy an off the shelf isolated DC/DX converter.

There does not appear to be many charge-pump voltage dividers that can handle your 24V input. If lowest cost is more important than power efficiency, use a common shunt regulator like a 7812 for the +12 rail. Use a charge-pump inverter for the -12 rail. Many inverters will will easily supply the 20mA you need, pick one with a maximum V-in greater than 12V.

If a floating ground is tolerable, you'll only need the 7812. It's output would be used as the ground for the driver. If you choose this method, be very mindful of which points are ground referenced and which ones are floated. Mixing the grounds will very likely cause parts to emit smoke.

I searched for "high voltage charge pump" and found the LTC3260 which can handle up to 32V and outputs an inverted and uninverted regulated supply, up to 100mA.