Cannot use xwikivars.vm variables inside ssx

 I trying to use velocity inside a ssx in order to have rules conditionally on whether the user is an admin or not.

#if ($hasAdmin)
  rules...
#end

However this does not work: $hasadmin is null, as it appears xwikivars.vm is not loaded in this context (no matter whether the ssx is global or on demand).

Are there any reason to disallow access to those global variables inside ssxs? Are there limitations for that or is it simply a bug?