Is there any rule prohibiting drilling of holes through any plastic that surrounds a motor? A hole(s) drilled could be used for natural air cooling of the thermal fuse.
No, this is strictly prohibited. Please see <R15a> which is quoted below.
The rule you mention refers to vex modification of electrical components. Since when has plastic been an electrical component?. Does that mean that metal cannot be bent, drilled or modified because metal is an electrical conductor, and therefore a vex electrical component? Confused!
If the connecting tag of a line follower breaks off, does that mean it is modified and not able to be used? It could be possible to use velcro to assemble it to the robot instead.