Your requirement is unlikely to be implemented as greenscript shall not test the network connectivity when parsing the tag, because it might very well happen that at precompile time the internet is not available.
I'd recommend you to use the <script>
tag rather than greenscript to load jquery for your case.
BTW, 1.2.8b is an outdated version. Because playframework doesn't accept module update, you should follow https://gist.github.com/greenlaw110/2868365 to update your greenscript version.