It’s always interesting looking at modification-esque surgical procedures that have faded into obscurity as other techniques have replaced them. A good example is eyeball tattooing (yes, it’s safe) which fell out of fashion after corrective colored contact lenses were developed. Another example is cineplasty.
Muscle tunnel cineplasty is a procedure in which a tunnel (like a piercing hole) is formed through muscle and then lined with skin grafts. This allows you to form a permanent piercing through large muscle groups such as the bicep. In the past (and sometimes now) these were used to attach prosthetic limbs to the muscles remaining in the stump. Here’s a couple photos:
These days smaller externalizations of muscle are generally performed which then attach to various electrical controllers. For example, this short video (source), which you can download either as a Windows Media Video (1.36 meg) or as a QuickTime MP4 (0.98 meg meg).
In any case, these deep muscle piercings are certainly very plausible, although far outside of the realm of the average underground surgeon due to complicated steps such as the skin grafts.