I'm also in the middle of a Model M chop using copper tape with conductive adhesive. It works fantastically well and can be soldered to. Going for 60% using a Teensy. Having some trouble with the firmware, though, since the one I chose drives the columns and senses rows and this causes a bit of problem with the 2KRO membranes... I cannot use right Shift and V,B,N or M together and since I use right Shift all the time I need to change the firmware.
I've been considering playing around with capacitive sensing and using aluminium tape on the hammers, copper tape on a closed cell foam layer with a coating of Shellac for the capacitive contacts and magnet wire to connect them to each other (minimal contact area to minimise parasitic and stray capacitances), but I'd like to get the basic board working as a "normal" Model M first.
I thought about etching a board for the 60% ones with individual contacts on one side, with boss connections through to traces on the back with the controller. That way i could use some sort of 2 layer custom membrane without traces. i would still be able to sense each key, with only one contact. (think of it this way: conductive foil backed sheeting for the top layer directly under the hammers, then a regular non-conductive later with holes cut to add a separation layer, then a PCB with bare copper pads home-run back to the controller (after a few registers since there would be 80+ traces)
I have decided to focus on the 70% conversions right now, here are a few pictures showing where I am at now. I have the traces done with the conductive tape, and it does work great! I went ahead and left the membranes intact as far as output is concerned, so I am still going to use the M controller behind the teensy, but I see the advantage to just going with my own controller.
These are 2 of 4 I am making like this, I will be putting them into custom wooden cases. (or ateast 2 of them, I may try modding 2 back into cut down original cases)
If I get ambitious, I may do led backlighting as well...... decisions decisions....