Bootloader for Microchip PIC18F26K22

We are a small-scale chemical manufacturer, and due to the lack of reasonably priced commercial explosive safe electronics on the market, we have made our own control cards based on PIC18F26K22. Since we are handling highly dangerous hydrogen gas, we have made the cards to be below all applicaple limits in the ATEX standard - below 200 mA, below 10 V (no sparks), and below 1 mF total capacitance. The units communicate using standard Modbus via RS-485. A windows 7 computer works as master, and asks each unit for updates using Modbus/RS-485 one or a few times every second. The master has a program written in Python We are today using 3 of our cards in our plant, and need to use more of them. However, it is a very tedious process to walk around to the various units, open sometimes awkwardly placed cabinets, plug in the programming cable while trying to balance the laptop, manually change the unit number in MPLAB X, and then program the units. We would therefore like to have a working and safe bootloader that can re-program the units from a windows 7 computer after a command for software restart has been given to the unit. Attached You will find the current program, in which some code for the bootloader would need to be included.