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Every real part has resistance including your piezo current source. Every switch resistance is a loss in energy (I^2 R * t)=E and includes diode and transistor switches.

What you need, I think, is to incorporate MPT theory into your block diagram and use a goal of matched impedances to achieve it. Otherwise, it won't even work in a simulator.

Try to obtain and list the following;


1. Source resistance at Imin:max
2. Rectifier resistance table from datasheet plots using the absolute and incremental slope of the VI curve @25'C. Use a spreadsheet if you can't find it on the web.  It's the incremental resistance that you can try to match for MPT, (neglecting reactance for now)
3. Capacitor values C,ESR options , Choose a few
as if C is too large you won't get enough voltage 
Ic = C*dV/dt or have to wait too long, dt to rise dV.
4. Identify all other loads and where losses can occur.
5. Choose how much power and time you want to test then determine the best plan to harvest into some load.  
6. Estimate best and worst-case efficiency. At low currents of 100uA , you have choose parts with lowleakage resistance.