If said GF or similar plugin is in the theme directory (and you keep your theme updated by the WP update mechanism) could you not make a plugin that loads the 'up to date' plugin from the themes directory using the useful wp_plugin_dir() functions?

Basically without even using the theme directly someone could enable the use of gravity forms (or similar) and use the most up to date version as the theme permits.

Entirely peculiar that 'free' has become as infectious as 'copyright' through use of copyright. I suppose it's just delightful that one does not have an edge over the other.