I verified for myself that the DC motor inside a V5 Smart Motor is indeed replaceable through strictly mechanical means (12 screws and the motor can be pulled right off its protruding contacts on the PCB), so there is no contention there.
While I began writing this post to justify my inclination to call this repair illegal, after carefully rereading the relevant rules, I am no longer convinced that it would be illegal.
However, I would still like to see an Official Q&A ruling on this if any team intends to actually use this repair in competition.
Relevant rules
<R21> No modifications to electrical components are allowed. Motors (including the internal PTC or Smart Motor firmware), microcontrollers (including V5 Robot Brain firmware), extension cords, sensors, controllers, battery packs, reservoirs, solenoids, pneumatic cylinders, and any other electrical component or pneumatics component of the VEX EDR platform may NOT be altered from their original state in ANY way.
a. External wires on VEX electrical components may be repaired by soldering, using twist/crimp connectors, electrical tape or shrink tubing such that the original functionality / length is not modified in any way. Wire used in repairs must be identical to VEX wire. Teams may make these repairs at their own risk; incorrect wiring may have undesired results.
b. Teams are advised to use the latest official VEXos firmware updates, found at www.vexedr.com. Custom firmware modifications are not permitted.
c. Teams may change or replace the gears in the "2-Wire 393" or "2-Wire 269" motors with the corresponding official VEX Replacement Gears
d. Teams may change or replace the gear cartridge in the V5 Smart Motor with other official replacement gear cartridges.
<R22> Most modifications and repairs to non-electrical components are allowed. Physical modifications such as bending or cutting are permitted and may be done to legal VEX Robotics Competition metal structure or plastic components.
a. Physical modifications to electrical components such as a legal microcontroller or radio is prohibited unless otherwise explicitly permitted, per <R21>.