Can you offer suggestions for this voltage interverting circuit?
I haven't modeled this but i would try higher switching frequencies.  If your not opposed to using a discrete IC you can use a 555 timer or similar to allow easier adjustment of the oscillator frequency.  Then again if you weren't opposed to discrete ICs I assumed you'd just be using an inverting charge pump or something similar in the first place... 

You need to fully charge C3 during the high time of the oscillator.  The value of R5 will also come into play here but generally speaking the faster you can charge and discharge C3 the more charge your transferring and the more current you can provide.

You can also move to diodes with lower forward voltage drop, properly rated schottky diodes most likely.  This will reduce voltage drop which has a couple advantages including being closer to a true inversion of Vcc and charging C3 faster by increasing the voltage drop across it when the oscillator is high.  Right now your limited to -(Vcc-2) for your output due to the two diode drops.  This could be reduced to something closer to -(Vcc-1) or lower with schottky diodes.  Note this is a max theoretical value, in practice there are multiple things in this circuit that could limit the actual output voltage.

If your not partial to this topology you can ditch the diodes all together and go with a more common charge pump topology, using transistors and a clock to control the charge/discharge cycling.  


  One thing I have noticed is it doesn't
  much like low impedance loads on it -
  the output voltage across the load
  drops by a noticeable amount compared
  with a high impedance load (that may
  just be me though).


Of course, you'be built an unregulated supply.  If you require constant voltage you'll need to add a regulator after this inverter.  There is also a limit to how much charge you can push through C3 which limits your output current.

EDIT-Replacing R5 with a P-Channel FET:


This replaces R5 with a complimentary FET.  Ideally Q3/Q4 are matched or complementary pairs.  You can get single packages with both mosfets inside, these have the best matching.  For this application they probably don't need to be perfectly matched unless your planning to really crank up the current.

Its possible that you will still want a small resistor on the high side (source of the p-channel fet) to allow control of the capacitor time constant.  Without any resistor the current spiking on Vcc could be high, only limited by Rd on of the mosfet.  I also don't know what kind of capacitor C3 is but some types could be destroyed by the current spikes without a resistor limiting it.