Under the hood it will store controller mappings in text files, yes, but for configuring there will be a GUI. I haven't thought about details too much yet.
About that MIDI2LR thing, I had a look at the MIDI protocol a while ago and it's trivial to parse, so I don't think I will need any foreign code. I might peek into GIMP's at most. The only thing I don't know yet is how to get to the Midi interface on OSX. But eventually we will figure that out, too.
Once the parts for tinkering are here I will definitely do posts on every bit of progress I make. Even if it's just a breadboard proof-of-concept thing at first.
And since Midi has a back channel It shouldn't be too hard to add slider movement.