The Ground Support Package will connect with an easy-to-remove umbilical to the main flight computer, and provide ground power, as well as debug functionality, in addition to computer-based emulation of any components or subsystems which are not attached, but remain critical for flight computer operation and testing.
A continuously adjustable, variable load. PWM output from the uC goes into an RC circuit, with R1 and C1 chosen to give a rolloff frequency such that the PWM output is effective analogized based on duty cycle. This input to the Op-Amp forces the inverting output to keep the voltage across the 1 Ohm Resistor constant. As a result, the voltage input to the Op-Amp becomes the forced voltage across the Resistor. Per Ohms law, the current flowing through the circuit must be whatever voltage is placed across the circuit in Amps. Thus, a 1V input results in a 1A current sink. The power is dissipated across the Transistor. Resistors R2 and R3 must be sized to configure the second op-amp in an inverting configuration such that the input goes from 0v to 1.1v (To fit the bandgap reference), allowing for measurement of actual power dissipated.
The Power Handoff Circuitry relies on a whopping big capacitor to supply power to the circuit. Assuming a maximum power draw of 3A, and a Maximum Voltage Drop of 10% Vcc, 6F is adequate to power the entire system for 1 second while the handoff is taking place. 6F of capacitance is $7 from Digikey, or $10 from Sparkfun. I'm not good at discrete electronics, so I'm not sure if the power dissipation across the Transistor Q1 would be too great to deal with. If it would be, simply use a relay.