I'd like to create Quark tomes for a few modded enchantments in my pack. In particular, I'd like to add tomes for holding, insight, and leech from CoFH Core.

Actually you can do this already if you go into the Quark config in game or out of game whichever you prefer. Find the Miscellaneous section then the Ancient Tomes section and under minecraft:lure in Valid Enchantments you can add more tomes simply by putting the (modid:enchant) so for example the modid would be cofhcore and the enchant would be holding, insight, or leech. (cofhcore:holding), (cofhcore:insight), (cofhcore:leech). If you find you don't know a enchantment you can go in game type /enchant (username) and then hit the tab button to autocomplete this should bring up a list of all available enchantments in your current game. Hope this helps!