This is a technique that I have used for many years.  You can tell from the photographs.  The key to the technique is the simple jig that I made from a dowel and some steel pins, Fig. 1.  Note that the center is hollowed out.  This allows better heat transfer when soldering and makes it easier to cut the wires out of the center after soldering.  The harness ring is made from brass tubing, Fig. 2.  The tubing was chucked in a lathe and the notches were cut using a fine circular saw.  Then the ring was cut from the end of the tubing.  The tubing is placed on the end of the jig and wires are strung through the notches and around the steel pins, Fig. 3.  The wires are soldered to the ring.  After soldering, the wires that span the center are cut out, Fig. 4.  After some final dressing with a file and sandpaper, the wiring harness is positioned on the radial engine with the soldered portion against the gear case, Fig. 5.  I have successfully used this technique for 1/48th and 1/72nd  scales.  For larger scales, I machine the ring with separate pieces for the wire exit points.

Wiring Harness Jig
wiring harness ring
Wiring Harness Rigged
Removing Wires
Wiring Harness Completed