Smarty plugin_dir?

Does Xataface have provision for a top-level Smarty plugin directory, in the same sense as it has a Smarty templates directory at $SITE_ROOT/templates?
As far as I can tell through scouring the codebase, the only plugin directory made available is the default Smarty one, which resides in xataface/lib/Smarty/plugins. I'd prefer, if possible, for my custom application to reside only in $SITE_ROOT, not within the xataface/ package, so I'd like to keep my plugins out of that provided directory.
So, is there something like $SITE_ROOT/plugins available? If not, could this be added?
As far as I can tell through scouring the codebase, the only plugin directory made available is the default Smarty one, which resides in xataface/lib/Smarty/plugins. I'd prefer, if possible, for my custom application to reside only in $SITE_ROOT, not within the xataface/ package, so I'd like to keep my plugins out of that provided directory.
So, is there something like $SITE_ROOT/plugins available? If not, could this be added?