Try this plugin[1] which will unsign Xcode and allows older plugins to work.
I also encourage you to file a radar [2] to let Apple know you're not pleased about them killing a thriving plugin ecosystem. Yes, I know there's a new plugin API but it's so crippled that 90% of the plugins on Alcatraz couldn't possibly move to it. If enough of us complain maybe they will listen.
[Update] Hmm, seems so. See this: https://github.com/inket/update_xcode_plugins