I'll bump this thread, not because I have an answer, but because I'd also like to find one myself. I actually contacted Graph Tech to ask whether I might be able to purchase one of their Ghost Resomax tune-o-matics without the saddles being pre-notched, but they said no. Here was their full response: "Unfortunately we do not manufacture any 12-string TOM bridges, nor do we offer any blank saddles. The slots in our saddles are formed in the manufacturing process and their depth-to-piezo relationship is very specific. I have had customer file and re-slot the saddles (at their own risk) but we do not endorse this practice."
It's a very niche product, and getting good response out of piezo elements small enough to have individual saddles for each string on a 12 string and still have enough output is difficult. The voltage a piezo produces is affected by the amount of deflection and the size of the crystal/film/etc. element. You make the element really small, and there won't be nearly as much voltage produced by the vibration. That means you have to add more gain in the preamp, which increases the noise in the signal. It's a non-trivial solution for a 12 string with individual saddles.
If you go with shared saddles, it's not as much of an issue, and you can just use a regular 6 string bridge with two notches filed in each saddle. Of course, that leaves the door open to intonation issues for the lower strings.