I think it would help to pass the $var (from $merge_vars as $var) so that a filtering decision can be made on the var by var basis.
Perhaps its overthinking it a bit, but some flexibility is better than no flexibility at all. Being able to only expose the SF fields that need to be exposed makes sense, at least to me :)
Aside from this and that (part of the first time working with it), this is a nice plugin. Thanks!