Unfortunately I was away from home and relying on tethering to a mobile phone for access to the Internet so using my home MQTT server for logging wasn't impossible, but was going to be inconvenient.
It occurred to me to give Adafruit IO a spin as it's designed for making generic IOT logging easy to set up.
Quick and easy to get working it has been logging just fine and I've not swapped back now I'm home. The dashboard is basic but all I need for this test.
In the first graph you can see the VPCC feature of the MCP73871 working well to avoid flatlining the voltage from the solar panel under load. It has a target voltage (arbitrarily set to ~5.3v here) and reduces current draw if the supply voltage drops below this. I've added a small trim pot to my test board to allow me to tweak this and do some runtime comparisons.
VPCC isn't MPPT but it will get better efficiency out of the panel than simply drawing until it can give no more and the voltage sags below a useful level. In the second graph you can see how without VPCC set the load on the same panel causes cycling like this, which also won't be doing the LiPo cells any good.
I have an idea ruminating to use an I2C controlled potentiometer along with a current sensor to make adjustments on the fly to try and optimise the solar output but it may be a minimal return compared to doing the tests once and picking a set value for VPCC. It will certainly increase component count.
There are packaged up MPPT solutions around, but there seems to be a gap in the market with them either super-efficient energy harvesting for tiny panels, or for large domestic/industrial power solutions. My scenario of ~4W panels with a ~100mA load isn't something I've spotted a good packaged solution for.
It wouldn't be impossible to roll my own MPPT boost convertor in front of the MCP73871 but like I said very likely to involve diminishing returns over careful use of VPCC.