I don't remember all details, but a quite simple and effective way to get +5V from a RS232 port was to abuse the charge pumps of a MAX232 type driver.
Use some diodes to get +10V and -10V from the three available lines, connect them to the +/-10V pins of the MAX232, with a resistor and a zener diode create a little startup helper voltage at the +5V input of the chip - once the charge pumps run, they also work in reverse, supplying 5V from the +10V or -10V, whichever is available.