Indeed, only one other person has ever bothered to post here. :) Welcome!
Also, the IRC channel is usually somewhat quick to respond but it must be a timezone thing. I've come back after you've left twice now. :)
I've been kicking around the idea of default/site-wide data providers but it has not yet been implemented. The downside of these would be that if you make a change to any post it would cause the entire site to rebuild. Right now I'm going to great lengths to try and be more efficient with the rebuild process (for --watch) and only rebuild things that actually changed.
Even as-is, updating any one post requires rebuilding any of the post pages that "use posts". If you were to use posts globally (or anything that depends on them them like the taxonomy data providers) it would require building the entire site.
There is no easy way to do this now. As a short-term fix you could consider creating a tag or category index page (in addition to the one already there) but have it output JSON instead of HTML/XML. You could then include it via javascript in your layout and load it dynamically that way. Maybe not the best solution but it would hopefully get you a little closer to your ideal layout.