barrel connectors which guarantee outer contact before inner contact?
In direct answer to your question, I don't think there is. This kind with an all-metal ring around the outside has a chance of working. I'll let you shop for whatever style it is that you need. 



You have an interesting problem. To quote the Wikipedia page: 


  RS-232 drivers and receivers must be able to withstand indefinite short circuit to ground or to any voltage level up to ±25 volts.


So, one might guess that a 24V connection would be guaranteed to not cause problems, however it appears that (your?) RS-232 level converter is either non-compliant or non-compliant with power off. Charging your power supply capacitor through protection diodes is probably not conducive to the health of the chip. 

The cheapest solution is probably to use a different chip and/or put Schottky diodes  from the output/input to the +/- RS-232 supplies and perhaps 12 TVSs from the output/input to your circuit ground. 

If money is no object, then a DC-DC converter on the input power is another solution, or opto-isolate the RS-232 as @Ignacio suggests above.