Years ago there was on the market a simple little oiling kit, a little plastic moulding and a couple of “U” clips. They may still be around but I’ve not seen any for years.
The moulding was designed to fit around the bowden cable outer after removing a small circle of the covering to reveal the wound outer cable. Once fitted the two “U” clips had to be inserted in the holes provided in each end of the moulding and then carefully crushed with a pair of pliars sufficiently to firmly grip the cable. The moulding was provided with an oil-can friendly hole that included an attached sealing plug.
If fitted near the centre of the cable run a weekly squirt of oil was all that was needed for smooth brakes and clutch.