I had to change a to input, but that worked in the template tester. Unfortunately I can't get Calibre to run from source anymore so I'm not sure if it actually works in the column itself.
EDIT: Until I fix the source issue (posted in development about it), I just subbed in my old code and put $#kobobookmark into the OEBPs test. That worked.
ANOTHER EDIT: Installing the Calibre beta lets me run from source again. it works. Actually since the beta also includes the recursion fix I can just use that.