How would one go about promoting a Area 51 proposal on another SE site? Should I form a question out of the promotion and post it? That seems a bit odd.
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I can understand wanting to share your enthusiasm for your favorite proposal, but advertising them on a meta site — even if it is veiled as a question — would not be appropriate. That is not the purpose of meta; Off topic.
We have already started grouping proposals into "Categories" so we can advertise them to sites that might share an interest. Beyond that, you should be going out to the wider internet to evangelize your favorite proposal. Cross-promoting proposals within the network has to fall to the developers through whatever means they can come up with.