This didn't quite work as VPCC reduces the load on the supply but as the voltage from my bench PSU isn't actually sagging it keeps reducing the current until it hits zero because the voltage never recovers.
With very little daylight on an overcast winter day I resorted to an old school halogen spotlight playing directly on some solar panels to test how it really behaves.
Starting out with a fixed resistor network for VPCC I could see that it was doing some form of load regulation but not what difference it makes as it varies, so this got changed for an old guitar potentiometer I had kicking around.
It's very tricky to set by tweaking the pot. As you change the VPCC threshold for the voltage from the panel it does change the load on the cell slightly but beyond a very narrow range, it stops charging.
Grabbing the data once a second I put this into a spreadsheet and did a scatter of the power provided vs the voltage from the panels, which is moderated by the VPCC threshold.
The main outcome of all of this testing is it's shown me that VPCC has little beneficial effect with the panels I have and had I set it by instinct it would have severely compromised charging. Bypassing VPCC and allowing the MCP73871 to draw as much power as it wants lands them in the 4.5-4.7v range which is where they appear to be most efficient anyway.
The other outcome is I have ordered some larger (165x165) 6v panels as all this was making me come around to the idea I just need larger ones. This may help offset things further so the dark evening doesn't leave such a charge deficit overall.
Interestingly during this testing when charging from USB I'm seeing one bit of behaviour that doesn't match the data sheet.
In the datasheet it says..
The MCP73871 device USB-Port current regulation set input (PROG2) is a digital input selection. A logic Low selects a one unit load input current from the USB port (100 mA) while a logic high selects a five unit load input current from the USB port (500 mA).
In practice with PROG2 high I see it max out at 400mA. No matter, I won't often see this as an issue with the little solar panels I expect to use in normal conditions.