A lot of people wire it like you have for a cleaner git look, but others, myself included, prefer the convenience of a vol pot at least and sometimes a tone pot, both on the git. Now I'm including a link, not saying you don't know how to do this, but you might find this article interesting(diagram 1b in particular), it shows 2 tone controls wired to control the treble and bass, might be what your looking for if your piezo a little hot and twangy.
Simplest way is to add an in line pre amp, you can hang it off your belt or put it next to the pignose, makes the piezo tameable as well.
Sean, you might like to check CB Gitty's piezo rod with equalizer pre amp. Shane has a video showing this rig built directly into the git, and a second video showing it "hacked" into a small cigar box. Both methods are very portable, and @ $17.00, it is certainly priced right. I just got one, and I"m building it into a stand-alone box this weekend. I haven't been a piezo fan - more of a mag pickup guy, but I can be led to new processes. So I'll be building some rod piezo gits to make use of my new pre amp. Good luck with what will certainly be many more builds.
Lots of good advise here. I like the sound that comes with a preamp, but I always want to find the right passive solution. Maybe someday after a lot of experimentation I'll find the right combination.
If you have an pignose HG 20 , a speaker upgrade to an eminence 6.5 will make a big difference in tone. I would abandon the piezo and install a humbucker if possible. Much more pedal and amp user friendly.Just my opinion.