I just put
program: columnupdate_seriesindex(0) in the template tester and it worked. The downside is I was testing with a book I hadn't wanted to change to 0. Oh well.
For future reference, can other called python templates update columns directly without the action chain?
Additionally: Other than the formulas action, would it work if called in a SFE template mode? That way I could do an if-then.