After some digging, I found an interesting USB bootloader fitting in only 512 words for PIC16F1455:

https://github.com/majbthrd/PIC16F1-USB-DFU-Bootloader

For this to work I have to add to the board a pushbutton on RA3 (the same pin as reset) and thus rrearrange the board a bit; after that I will send the board in production.

Stay tuned!