Good call. I hadn't noticed the lack of channel. Surely Shimano must have thought of this, no? Further, I notice sloar's braze-on unit is different from the one in the video in some subtle ways. Notice the topmost rounded part of the front derailleur that contacts the frame braze-on (I wish I knew the proper nomenclature), it's different from the rounded section of the clamp-on unit where it meets the clamp (more angled round at the end). If designing a braze-on mounted FD, wouldn't Shimano account for the cable routing? Have you tried simply running it through that gap between the fixed fd body (below the limit adjuster screw) and the swivel arm below it, then out & around the pinch bolt?

Also, you say the arm strikes the braze-on tab. Maybe a picture of that contact point? Just a thought: If you start with the lower limit lower, will it give you extra range on the high end? Also, failing the rest, it's possible some judicious filing can gave a bit of necessary clearance to get int he big ring.

@carbomb:follow? (not sure I'm helping either, to be honest.)