Stick an electric guitar string on the underside of the skin then a magnetic pickup under that. Don't cut the skin. It'll work.
Oops. I remember a similar, open discussion in the FB Group where I suggested a small light piece of steel or iron wire on the underside of a wood box or skin when someone didn't want to cut the surface. Pretty sure it was a box. And an Aussie named Jef mentioned a guitar string as fitting the trick.
I have used the iron wire since I got a spool of it around and I like the way it works. It also surprises me how very little movement is needed to produce a clean tone from a mag pup. We see the huge vibration from the strings on a typical electric guitar and think it must be necessary in order to get any sound. But I built a tin banjo with the bridge sandwiching the lid and the pup together firmly. Still get good volume and almost no feedback unless the gain is way too high. Point is that it doesn't take a lot to distort the magnetic field enough for the pup to work well.
As a side note, I have also seen a banjo with a single coil from a strat under the head.
It can be seen in this video (she is also a very interesting performer) https://www.youtube.com/watch?v=1NhsVwWQp2M
I kind of agree with Oily here on not adding a pickup, but I'm kind of a banjo traditionalist myself. I bet it will be loud enough to keep up with a couple acoustic guitars.
I'm a big fan of piezos for CBGs and can coax some "natural" acoustic tone out of the right box, but I haven't had the same luck with skin heads. I've built two, and they both sounded pretty crappy even with a lot of dampening of the piezo. The tight skin is pretty unforgiving when it comes to handling noise.
This mag pup trick is very intriguing though. Anybody got some sound samples of this in action?
And do I have the scars to prove it. The ironic thing is that skinheads are usually amped up.