Is it possible to generate RF to then harvest instead of just using background RF?
Wireless charging for portables has largely settled on the Qi protocol. Qi works at close range - 4cm or less. Qi could probably be hacked/modified to use a directional emitter to increase its range. More about Qi here: https://www.electronics-notes.com/articles/equipment-items-gadgets/wireless-battery-charging/qi-wireless-charging-standard.php
Xiaomi has shown a long range charging system, but it's not on the market yet. More here: https://gizmodo.com/xiaomi-announces-a-long-range-wireless-charger-you-prob-1846158088
And finally, here's a chip that does generic RF harvesting: https://www.powercastco.com/wp-content/uploads/2018/06/PCC110-PCC210-Overview-V1.6-ONE-PAGE.pdf. There's several versions that support different ISM bands.