Canon DSLR bodies prevent poor auto-focus performance by disabling it completely when the lens reports maximum aperture at chosen focal length smaller than f/5.6. This implies that some lenses will not auto-focus on a teleconverter, e.g. the popular Canon EF 100-400 f/4.5-5.6L, or any f/4 lens on a TC stronger than 1.4×.
In an answer, Matt Grum mentions that
There's a well known trick whereby you tape the pins on your teleconverter to prevent it from reporting the true aperture of the lens+converter combo so that AF remains enabled.
Which pins have to be taped? Are there any other consequences than just enabling AF, e.g. how will that taping affect focal length stored in EXIF info?
If there are any other workarounds, e.g. using alternative firmware, those options would be welcome too. Let's skip the obvious option of moving closer and removing TC :)