Have you considered adding any special connection ghosts to any of the main characters of the Lockwood & Co series? Actually answer either that or this: Will you ever crossover at any point the Bartimaeus novels with Lockwood & Co? Like maybe a villain summons a djinni or something & Lockwood & Co go through the books to find another djinni (voila!- Bartimaeus) to fight back.
Ooh, good questions. Well, I'll answer the crossover one. I think not: it's tempting to have some kind of crossover between the two universes, but if I did that it would at once undercut all the rules that govern both series. The funny thing about writing fantasy is that it has to have cast-iron rules: if you deviate from them, then anything can happen, and the reader straightaway loses interest. I did once have George reading a comic about a genie, but that's as close as it went (and I may have even cut that!).