I don't know of a commercial device like what you describe (it sounds very much like a sheet film version of a 35 mm databack), but it should be relatively simple for someone with electronics skills to design and build. You need an LED segment or dot matrix display that can be put in contact with the emulsion surface of the film -- if wired right-reading, it'll produce an image the same orientation as the photographic image on the film; if mirrored, it will produce an image that reads correctly from the emulsion side. You need a very basic logic/memory system to accept and hold the data to be displayed, a duplicate display to show what you've entered, and a (presumably adjustable for exposure) momentary switch circuit to light the contact LED for a moment to expose the data onto the film sheet. If I were an electronic hobbyist, I could probably build such a system with components available from Radio Shack (or a store such as Radio Shack used to be) for $100 or so in components (including a computer-on-a-chip, most likely) and 4-10 hours of working time (plus software development, if a computer chip is used, but devices similar to this predate modern microcomputers by several years, using 74-series TTL logic circuits and slightly larger integrations). Built with versatility in mind, it could be used on the corner of any size sheet (or, for that matter, on photo paper -- one reason one might want to wire the contact display for mirror image and support variable exposure).
Alternately, it seems to me it might be possible to hack an existing databack to do this job -- if you get a camera with databack for a good price (perhaps one that's got a damaged shutter or lens mount), it might be cheaper, though removing the LED unit from the camera back and getting it to work in the right manner might take longer than building the device from scratch.
