cable guide for new derailleur
 

This bike currently has a front derailleur with a housing stop oriented vertically so that over time enough grit accumulates to cause the cable not to slide all the way back when shifting down, so that I find myself sometimes resorting to unclipping and tapping the cage over with my foot. This really doesn't take more than several hundred miles to develop, and so I am looking at replacing the derailleur with a less troublesome top-pull version.

https://cimg6.ibsrv.net/gimg/bikefor...92331eeb34.jpg

Suppose this vintage cable bracket would be a tried-and-true way to get the cable routed .. think they are around $30 on ebay.

https://cimg0.ibsrv.net/gimg/bikefor...e42fcdbeec.jpg

I would also like to consider using a guide mounted below the bottom bracket shell for a cleaner / less vintage look, though most I've seen require drilling and tapping a hole for the screw. Has anyone had experience installing one of these with strong transfer tape, thinking of the stuff used on USPS flat rate boxes. The guide would likely never slip around, especially with the tension of the cables assisting the adhesive.

https://cimg4.ibsrv.net/gimg/bikefor...fd9236ede9.jpg

Replacing the current cable guide with an under-the-bottom bracket type will allow you to eliminate the current quide and also that section of cable housing that's causing you the problem. As you surmise, cable tension is sufficient to keep it in place and double-face tape is more than adequate insurance. However, it appears your rear derailleur cable is also routed above the chainstay and that will be more difficult to change as the rear cable stop will have to be replaced with an under-the-chainstay type.

AS stated if you went under with the plastic guide and wanted to change the rear there as well you can add a clamp on stop to the chainstay. Still probably cheaper than the other clamp on bracket.

https://www.ebay.com/c/1931733187

NIB/NOS Campagnolo #636 Chainstay Cable Housing Stop Clamp for Rear Derailleur: Chrome Steel - 5/8" Bolt/Nut (Many Available) - Bike Recyclery

You could also use hooded ferrules such as these Hooded End Caps | Jagwire to keep the gunk out. Do a search or check your LBS if you need less than a jar of 30 of them.

Wow, had no idea about the hooded ferrules -- not sure if they would work flawlessly in a vertical position such as this, but thinking I am partial to replacing the derailleur anyway.

https://cimg7.ibsrv.net/gimg/bikefor...959424e4ee.jpg

So this is a cable stop but also has a perpendicular race? Curious what functionality it has beyond this other one?

https://cimg0.ibsrv.net/gimg/bikefor...57a5e283a0.jpg
Even so, the cable for the rear derailleur would preferably maintain the same route -- it would just leave a vacant race under the bottom bracket. The tension on the guide would be more balanced with two cables, but hopefully I can stick it well enough. If necessary, drilling and tapping a hole would not be difficult, especially with a sealed bottom bracket easily removed.

That chainstay housing stop is designed to be mounted so the cable runs above the stay. The perpendicular piece is to catch the chain in the event it derails off of the crank.

Are you sure? Most of the time when I have a derailleur shift travel issue it is the derailleur needs the cleaning.