Actually, now that I think about it, there’s an existing Kosmo module that I think would do almost exactly what you want. My Dual Quantizer. Set it to quantize to octaves, put the pedal CV into CV1, put the CV you want to add it to into CV2… well, CV2 has an attenuator which you don’t really want on a 1 V/oct signal. But maybe if you left that out?
But you’d have to get the board and panel fabbed, and it does have an Arduino, and it does way more than what you need (for starters, it has two quantizers), which means there are a lot more components than necessary, and at 10 cm wide it is kinda chunky for this job.