First, this forum seems certainly like the appropriate place to discuss anything Spring Social, even the Spring Social Grails plugin. That said, Spring Social Grails is developed by a community member and as such I don't know much about it (played with it once or twice, but am far from an expert on the topic) and I don't know if there's anyone on this forum who knows about it.
In general (Grails or not), there is a plan to integrate Spring Social and Spring Security closer together and there has been a handful of community-led attempts to do that. In short, it's on the roadmap, but we've not yet begun to go down that road. Now, how that would play into a Grails app, I cannot say. If anyone is interested in helping move the Grails/Spring Social/Spring Security story further along, I'm eager to see it and will help out as much as I can.
But back to your original question: Is anyone else on this forum aware of a spring-security-social plugin for Grails?
Craig Walls
Spring Social Project Lead