Yep if possible I'd do that too - honestly these small little things I didn't look for in the test drive as they are so fringe case - but if the head rest moves up /down a little to get a hanger in there, yep that could do the trick!
along the lines of a tie down - big fan of a cargo net which requires tie downs to clip into (at least in my current car that is how it works)
I didn't notice in my test drive but I guess there is no handle bars to grab onto on the roof above the window. If that is the case, how can one hang a...
looks like a roof shade is already available:
https://www.evbase.com/collections/rivian-r2-accessories/products/glass-roof-sunshade-for-rivian-r2-ice-crystal-sunroof-shade-foldable-heat-reflective-sun-visor