Thanks Ron. Overwriting 0.4.1 (which was dev when I put it in place) with 0.4.1.1 now allows manual post updates for both blogs to show in the Tags blog. The Populate Posts attempt from the missing blog, however, resulted in:
Fatal error: Cannot use object of type WP_Error as array in /usr/www/users/[username]/[wp3location]/wp-content/plugins/sitewide-tags/sitewide-tags.php on line 194
so I'm into manual updates through the Edit Posts pages.
What I don't understand, though, is that the posts from the missing blog are still all in the Primary Blog posts table in the database. What would prevent them from showing if they've already been pulled; whether the plugin was activated or not? Seems that, once they're in the posts table, they'd show, no matter what.
While the Featured Image thumbnail came across fine, one image in the body of the post that came back doesn't show. It's internally linked, without the "http://domain.tld/" prepend in the img tag. I thought that this didn't matter in recent previous versions. Did it?