Mightier Than the Boom

Mixer Chris Durfy has put together a detailed how-to on planting a lav mic inside the body of a pen:

"On my last show, I was having some problems with body mics and scratchy police officer wardrobe.
After trying different ways to mic up the cast with hidden techniques that were not 100%, I [thought] I’d venture into the realm of the pen mic...

This mic is great for cast with pockets on the shirt. Cops, scientists, etc… You will need to get together with the wardrobe department to have them cut a hole behind the pocket for routing of the wire to the pack."

Link.