Do any DA's Offices use their own investigators as fingerprint experts, for pen packs, prior judgments, etc, in trial?

Collin County does for priors and such. In Dallas Co., the Sheriff had two deputies who were qualified to do that. That separation is a good thing, in my opinion. But, having those guys in your office can help you catch priors with problem prints earlier in the process.

We use an investigator in our office and a detective from the SO. Our investigator takes a look at it and lets us know if we can make the match and takes care of cases where there is truly a question of identity, and then we have the detective available if there's a question about the separation of duties issue.

Our office has used our own investigators to prove up pen packets, etc. through fingerprints since 1984.

I have an investigator certified to do prints. I use him as a backup in the event the SO witness is not available. Pen packs are pretty simple matters if the packet has a photo of the defendant along with the prints. I don't think using an investigator as a witness in this manner is subject to a built-in bias argument any more than an investigator for my office testifying on any other issue such as taking a confession, etc.