It turns out that once the grooves are established, they will remain pretty straight even with hand held scraper.
For my test pieces, I used a variation on the method that Adam Karpowicz mentions in his book on making Turkish bows. http://www.ottoman-turkish-bows.com/ That is to use a straight edge and a scribe to make one good straight line in the horn and then use that line to guide the first tooth on the scraper. By gradually bringing the scraper to level, the other teeth are brought into play a little at a time. Once the grooves are established on one side of the guide line, you do the same thing by moving the scraper over and gradually initiating the rest of the grooves.
Once the grooves are all there, it's just a question of continuing to scrape until they reach their full depth. Most modern horn-bow makers use some sort of guide to establish their grooves, and then remove the guide for convenience. They are either using a tool like this one. https://img.photobucket.com/albums/v709/Smokeys/TorysCamera002.jpg or else just holding the scraper blade in both hands like this.. http://i.ytimg.com/vi/IZ2lzEhvRro/maxresdefault.jpg The limiting factor here is getting enough force on the tool. As a result, these guys spend hours grooving the relatively small amount of surface in a hand bow's limbs. A horn crossbow is going to have 5 or 10 times as much grooved surface as a hand bow does. Using the technology that the modern guys are using would be a real source of repetitive stress injury as well as taking a very long time.
I am hoping that my proposed machine will allow a good, powerful, and comfortable two handed grip while still maintaining the truth of the grooves.
(Note: I revised my estimate of the total grooved surface comparison between hand bow and crossbow. )