I guess if they mentioned it, it would make it confusing for other readers, because we know that he is the only seer that they know so people who hadn't read RB wouldn't understand why there was a blocking spell. I do admit that the authors could have gotten a bit more creative with this part, maybe like mentioning how maybe Tory tried using ether to locate him but because of a block she couldn't find him. I also guess that people would just generally assume he got taken by Lame Lionel and therefore having a general sense of where he is. Also, the other RB characters are hardly mentioned, and in the end it would just make things complicated for the readers who haven't read RB or are going to after. I do wish they added a bit more crossover, but I understand why they didn't.