I have not heard that. What makes the rifled 12-lber you speak of "look like James Rifles"? Is it because they had 15 lands-n-grooves? Seems like an enormous effort for something that 99% of the park visitors wouldn't even know was about.
There are a James shells cast in the 12-lber caliber. I've only seen fired examples which don't have the sabot so I don't know what rifling was used when they were fired. The shell isn't in Jack Melton's book or the Dicky-George book to see what they thought although Pete could chime in here if he has any insight to this question. It is in Col. Biemeck's Volume IV book. These books by the Colonel offer a ton of information (and all serious collectors should get at least volumes II, III, & IV) but unfortunately for this shell there is no offer of what gun it was fired from. I recalled that our own Jack Bell has a table in his Heavy Artillery book showing the known cannons and the number of grooves they have. Fortunately he lists the rifled 12-lber! Unfortunately it doesn't answer the question because there are 8 different answers from 3 to 18 depending on the maker. FWIW, none of them are 15 but because the 3.8-inch rifle used 15 doesn't mean James would specify that number for the 4.62-inch gun.
