I've just found a bonus EvS mention in one of the short stories I'm reading, 'Biggles Learns Something' from Biggles Flies to Work.

"We know him all right," replied Biggles, grimly. "It's our old friend Laxter. You remember the ex-RAF type who at one time was acting as second pilot for von Stalhein."
"Do you think von Stalhein is behind this racket?"
"No. The money would be chicken-feed to him."

That's the extent of it, but it's the first time I've seen him mentioned anywhere outside of the EvS books. This is in a story about how criminals are being transported out of England by airplanes and helicopters that are allegedly flying wealthy patrons to Ascot. There is one thing that makes me wonder if EvS was involved elsewhere, because the story starts with an anonymous tip-off that a man who's wanted in France was seen in a TV shot of the crowd at Ascot, and if there's one thing that EvS specialises in, it's the anonymous tip-off, so perhaps he got wind of what his former colleage Laxter was doing and had some reason to want to use Biggles etc to interfere with it.

Intriguing mention of EvS being only interested in really lucrative work. The short story was originally published after Buries a Hatchet, so either WEJ is intending it to be set some years earlier, or else Biggles etc are still wondering if EvS might be reverting to his criminal ways again. If this really is set post-Hatchet, then I am completely certain the anonymous tip-off was EvS, that is exactly the sort of thing he'd do and also he's likely the sort of person who would watch Ascot on TV. If it's set back in EvS's criminal days, then it still could be him but in that case it would be because Laxter is causing him some kind of problem and he wants to get rid of him by setting Biggles on him.

And I don't recall Laxter from any of the other books, so either I've forgotten something, or else this is a reference to an unwritten von Stalhein story. Interesting that he would specifically want to hire an ex-RAF pilot; I'm going to go ahead and assume this was part of one of EvS's attempts to form his own team and he thought maybe the RAF was the secret sauce that made Biggles's teams work.