Based on the last update from the OP, there's not actually any need to switch on the "load" using a controller, as the load is the controller. It's simple just to have the solar cell connected to a large capacitor, powering the arduino via an LDO regulator, and the arduino using its brownout reset circuit to protect from under voltage. The reed switch is connected to the arduino input, and can use an interrupt from sleep (if desired) to decide to use the transmitter.
The capacitor needs to be sized appropriately to supply the transmitter for its single broadcast. A battery can replace the capacitor, if desired.