How do you reprogram an XMEGA after you have disabled the `RESET` pin using the `RSTDISBL` fuse?
The PDI functionality of the pin is still active on an XMEGA even if the RESET functionality is disabled via the RSTDISBL fuse. You can program the chip using a normal PDI connection. 

How does it do this? There is a dance between the PDI clock and data pins that puts the chip into programming mode without needed an external reset. From the datasheets:


  Enabling
  
  The PDI physical layer must be enabled before use. This is
  done by first forcing the PDI_DATA line high for a period longer than
  the equivalent external reset minimum pulse width (refer to device
  datasheet for external reset pulse width data). This will disable the
  RESET functionality of the Reset pin, if not already disabled by the
  fuse settings.
  
  Next, continue to keep the PDI_DATA line high for 16 PDI_CLK cycles.
  The first PDI_CLK cycle must start no later than 100μs after the RESET
  functionality of the Reset pin is disabled. If this does not occur in
  time, the enabling procedure must start over again.