Unless the burden resistor is switchable in/out of the line (perhaps as part of the 3.5mm socket?)
Ah yep. I had a different picture in mind because the specification was for a specific CT. The kind of flexibility I was hinting at could apply to any input that’s true.
I’ve been thinking about this thread the last couple of days. I wanted to add to my first post (not sure why I didn’t) that it looks an interesting project and looks well designed! I’m no expert, but have done enough with my board to know it can be very hard work making design decisions, and I trust you understand why I just don’t roll out all the different ways of making inputs flexible, because most of the fun’s in figuring out these details for yourself 
Good luck.
(I/we will look forwards to some pics of progress I’m sure…)