5V I/O Chip Which Works Like An Atmel µC Port?
You could implement a memory-mapped peripheral into a 5V CPLD reasonably easily.

I was going to suggest the XC95 series, but Element14 and Digikey are showing no availability for 5V parts.  They do have a reasonable range of 5V CPLD's from other vendors.

If you're not familiar with Complex Programmable Logic Devices, they're the little brother to the FPGA.  They have a very different internal architecture to FPGA's, they tend to have a lot less capacity, and they usually store their configuration in internal EEPROM or similar instead of in an external Flash.