I definitely think leather headliners would be way too much. Like someone else mentioned, Alcantara would be very appropriate, though. I can't wait to see this interior completed; the quality of the leather & the stitching are on par with modern high-end German cars for sure. I'm thoroughly impressed with the lengths to which you've recovered everything... this will truly be the only DeLorean in the world with a full leather interior.
Are you having the steering wheel redone as well? My second DeLorean had a refurbished steering wheel with Nappa leather & beefed up padding to make it slightly thicker. The original steering wheel leather (especially if it's not aged gracefully) is one area many DeLorean owners seem to neglect when tackling full restorations. The steering wheel leather tends to shrink & pull away from the center medallion; not to mention it's grain isn't the best, either.