Sorry, that's a "Premium" feature. In all seriousness, my XSE Premium has a dedicated button to switch to 360/front camera. I don't recall if it lets you check the rear cam as I don't use it that often. It's annoying that modern cars have several cameras but they can't be used on-demand to help with visibility or as dashcams.
Dang. I'm hoping against hope that some fanatic has found/will find the magic pushbutton sequence, inCANtation, or engineer a hack to do it. In the meantime I left my humble request for product improvement with Toyota at: Toyota Customer Service