I prototyped it on a small piece of prototying board, and it's a simple enough circuit that I'd encourage anyone who wants to give it a try. The JFETs could be just about anything with Idss of 1 to 5mA and cutoff voltage of 1 to 3V; I used 2N5457 because I had some lying around. (The schematic reads MMBF5457, which is the surface-mount version of 2N5457.)
On inspection of the circuit you'll notice that R6 and R7 form a voltage divider, so this preamp actually has attenuation rather than gain; that's necessary because, with the piezo properly buffered, it's got so much signal that it would otherwise be louder than the magnetic pickups. If you are building this preamp for a different bass and don't need to blend with a magnetic pickup, you might want to use a higher value for R7 (such as 100k), in order to get the maximum possible signal.
Those same resistors also affect the interaction between the pickups. In the middle blend position, each pickup is loaded down by the impedance of the other one. This is good, because otherwise the midpoint would be twice as loud as the extremes, and also because it helps provide tonal range. If you want, you can reduce the effect, by increasing the values of the resistors.
The pictures and schematic here describe an earlier version of the PZP-1, and are mainly of historical interest. The latest version of this circuit has adjustable output level and is built with surface-mount components. If you're considering building one, I'd recommend that you use the latest version of the circuit (whether or not you use surface mount, of course, is up to you).
A photo of a prototype
And here it is as a finished unit. The knob is to show scale; the board measures 1.2" x 0.8" (30mm x 20mm), and with components it is 0.4" (10mm) thick.