Right now we are at over 12k of codebase, pretty close to the point where things will start flaking out. I think if we try and do elm AND iso AND injectormonitor that we will definately break.
The one thing I can think of to keep the codebase together on the reusable bits (lcd and buttons) and allow us to have ifdefs to minimize code size AND still be cut and paste, is for me to put a bit of php (or shtml or whatever) together that will basically prepend a #define to the file depending on url parameters to say (Oh, the user clicked on the elm327 version).
The other thing to consider though, is that all these ifdefs are going to make things even more incomprehendable for your beginner programmer. Maybe the php thingie needs to stitch the actual code together? Anyway, I'll be happy to solve it, whatever it is, when there is a real reason to do so (i.e. here's a second working thing that we actually need to maintain)
re: ISO hardware, keep the LCD pinout and buttons the same for now, and keep the additional hardware to an absolute minimum (couple transistors and caps, etc) and use all through-hole if possible.