Yesterday I sat down to this project and wrote the firmware. At first I wanted to write just the interpreter alone and leave the display, keyboard and UI tasks for later, but I got so immersed into it I've done it during one long night. Coding in assembly language is great fun.
At first I programmed the interpreter alone - the interpreter was debugged just in MPLABX simulator, not running on real hardware at all. Along with a short demonstration program it fit exactly into 256B of code, so I felt confident to continue. LCD support took another 100B, most of the code lies in editor and UI. It's all about 600 lines of code.
You can enter both the code and data input (editor has two modes - you can work with full ASCII charset or just 8 BF symbols) into BF interpreter - in "Hello world" the input obviously isn't used and interpreter doesn't fetch a single byte from it.
Including two sample BF programs - the code size is few bytes short of 1024B limit, so I have still plenty of room for improvements ;-)
More details soon.