Would also be very interested in having ability to force failover from a runtime rather than only in the webpage status tab, which has other information I wouldn't want the person in charge of failover to interact with.

Proposal for a scripting function was discussed back in 2011. https://forum.inductiveautomation.com/t/question-about-the-redundancy-setup/2591/15

Right now, the option I have to programmatically force failover is to execute the "stop Ignition" batch file, which is a poor way of accomplishing what the "force failover" button accomplishes on the webpage.