My adventures in electronics recently took me to the old world of simple serial RS232 programmers for Microchip PIC microcontrollers (16F84A should sound familiar).
Most modern serial boards do not output full +/- 12V, but some voltage in between, which is still perfectly acceptable by the RS232 standard (+3 to +12V is a valid "0" logic level, -3 to -12V for "1"). But the lower voltage breaks those simple programmers.
While looking for a USB-based solution I came across an Arduino-based PIC programmer called Ardpicprog (currently hosted at github.io). The huge advantage of Arduino boards is that they offer a virtual COM port over USB, which works with modern operating systems (let's say from WinXP and newer).
The new circuit will cost 5 to 10 times less than commercial products (5-10€) and directly replace the JDM programmer in the current housing.
Writing the firmware to the uC will now require a different software, but for a once-a-year operation it is a satisfactory alternative.