I figured it out. Arc uses `lambda` from Mzscheme, and that version of `lambda` doesn't support keyword args. Trying to import Racket's `lambda` caused a conflict with the other `lambda`, so in the end I imported Racket's `keyword-apply` to do the job.
I have committed this code to the Anarki repo, so now everyone can use Racket keywords in their Arc programs. Yay!