It's kind of like the deal with pine cones. The growth that produces the cone or seedpod usually winds up getting pruned off as part of basic maintenance, just because the cones and seeds usually grow right out there on the extreme ends.
You would have to either overlook it or have some gnarly old tree that wasn't pushing real hard. I get seeds on my pre-bonsai, but rarely on the more manicured bonsai. I have a red pine with cones though.
