I'd ask the guy. Kickstarters I've backed have gone both ways, but PDF-only for stretch goals is fairly common.
It largely depends, I think, on who's authoring them. If it's the same author, it should go in the main book. If it's freelancers, you probably wouldn't want to hold up book production if they flake out on you. I'd guess PDF, since this would significantly inflate the page-count.
e: And Brust is apparently not writing it directly, but overseeing the authorship of the guide and everything. Which is pretty darn awesome, still. I'm a big fan of the series, too, and I'd be kicking in if I weren't already.
There's been a few! They are in the updates.
Here's the newest one. (backer-only of course)