My intention is to bring it to the notice of both, askers and answerers, who use the tags.
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Tags have a tag excerpt and a tag wiki.
The first is used to explain briefly what the tag is about and what questions it refers to (these two things can be achieved in the same sentence, too). The second is not directly visible and can only be read by exploring the tag. Here you can expand on the topic it refers to and other things.
I think that here you could link the proposal/site in case some questions are about that topic but are too specialized or in other ways not appropriate for the site you're in, in this case SO.
The link will be usually hidden, but at least this way you won't be too invading and the notice won't be considered as "spammy".
You might want to suggest an update to it's tag wiki. I think that's an apt location to mention a site proposal that covers the entire tag.
Any chat rooms that the active users visit would also be an appropriate place to talk about a site proposal...
One thing I would not recommend doing is posing about it in comments in and around posts containing the the arduino tag... Comments like this would become very noisy and distract from the main issue.