In writing historical fiction, is it ok to have fictitious things
happen to real people? Or can I expect descendents of heretofore
obscure personages to come out of the shadows to sue me?

This may be of interest:

http://ask.metafilter.com/mefi/18958

Also remember you might not be worried But your publisher will be.

If the people you're writing about are long dead (examples: Mary Todd
Lincoln, George Washington, Teddy Roosevelt), you'll have no worries.
Although authors often includes notes at the back of their novel,
pointing out major things in the story that are (or are not) factual.

On the other hand, if the person is living, or has children living,
you may run into legal woes.

Kriswrite

What civil action might a relative of a defamed deceased bring? One
difficult but not impossible route might be to rely on one of the many
state criminal statutes that make it a misdemeanor to defame the dead.
Most of the statutes are relics of the pre-federal
constitutionalization of the defamation law, but they are still on the
books, and can still be invoked

http://66.102.9.104/search?q=cache:6Sl7EqWAEkkJ:writ.news.findlaw.com/dean/20021122.html+%22Defame+the+dead%22&hl=en&gl=uk&ct=clnk&cd=7