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I'm in a very similar position as you except having a steel frame (with chainstay yoke) modified for Internal Di2. Just went round a framebuilder today and he will add a reinforcedhole at the top of the downtube, hole in the seat tube for FD cable exit and enlarge and existing drainage hole in the BB shell for RD cable exit. Internal junction B will either be in the seat tube or the downtube.
Did you consider the Ritchey logic-E headset that would allow you to bring the cable into the headtube without needing to drill a hole (if there is a vent hole opening inside)?
SoYaap-
I knew there were headset options out there but it all looked a bit complex. Either way, a guy was selling the same frame on ebay and kindly sent me a pic of the Bottom Bracket shell. Doesn't look like I could fit a Junction-B inside either the down tube or seat tube so unless there is a magic way to enlarge the holes, I'll probably not bother.
cjr
Was thinking of an entry point at the front end, hopefully room for a junction box in DT with access from the BB (big if) and an internal battery in the seat tube. Then a hole near the bottom if the seat tube to supply the FD and an external run to the RD.
Looking to switch over the Di2 setup from my Fairlight for some more off road fun over the spring/summer. Trying to do it with a cheap frame, once you get the up to date features, internal routing, flat mount, thru axle etc the frames start to get expensive. I'm seeing at as an experiment really.