Stirling would deny any cookies having been on his person. :P Â
Do you intend to have Peragrine as well, or just Stirling?Â
As for how they should meet him, I think the cookie idea is just fine, and as for the conversation thereafter, I would suggest that once James is shown to be commanding, Stirling immediatly questions his command, asking 'what makes you in charge?' Perhaps in the next few heated moments, it becomes apparent that James knows more about the situation than he can currently tell (due to being chased by currently undefeatable monsters) so Stirling sticks close to them in hopes of learning more.Â
Actually, that's more than just a cameo, so maybe not.
Wherever there's a battle, Stirling would be there. He is a Sentinal Samurai but also a sort of... warrior. He doesn't like fighting, but he has this need to destroy the evil, if he's able too. I really like a quote from Captain America in Civil War:
Stever Rogers: If I see a situation pointed south, I can't ignore it. Sometimes I wish I could.
Tony Stark: No, you don't.
Steve Rogers: . *pauses, then smiles* . .No, I don't.
So really, he could show up in any battle. Or defense.Â
Yeah, that's more than a cameo. Shhh...I'm not too smart. Yeah. It would have to be shortened from Stirling tagging along, unless he jumps, say, into an escape ship when they leave to Avant Gardens, and never really shows up again, until some later book when we actually do a crossover.