"RF energy harvesting" Can rf transmitter be a rf signal generator?
Yes, you can use a transceiver + amplifier for 915MHz. Your usable range will be very low (1m at most) or charging time will be very long.
The last capacitor's value is determined by your energy requirements. You will have to calculate how much charge you need over what period of time.
The HSMS line from Broadcom is obsolete. Good luck finding alternatives.
For stable signal, you can use UHF TV stations, they transmit at much higher power levels than your ISM transceiver.
The original RF harvester demo from TI (WiSP) used the same circuit with a high gain antenna aimed at a TV transmitter to power an MSP430 with a small LCD.