I just modified my SENA baseplate today, turned out better than I had hoped.
I pretty much did the modification found here:
The only difference is I mounted the 3.5mm jack INSIDE the SENA baseplate. I used a Radio Shack 274-246 stereo panel-mount phone jack ($2.69) and wired it to disconnect the internal speakers when the earbuds are plugged in. Everything else is untouched and fully functional (I did not cut any traces on the circuit board).
Sorry, I didn't take any interior photos. There's just enough room inside the SENA to mount the jack. I did have to cut out some of the plastic on the inside cover plate to get it to fit back together, but as you can see, it turned out great.