Great article with a host of practical circuit suggestions. I can see using this approach on a handheld battery-based instrument used in the field. A solar charging option is a perfect feature when you can't get back to 'base camp'.
It's a simple, elegant approach covered by quite a few papers. If it has a downside, it's that you do need to _know_ what the "maximum power voltage" parameter is for your panel. Maybe this can be acquired in production line calibration. Solar panels are rather variable from batch to batch, so you can't just fit and forget this sort of electronics to an arbitrary panel. The extra complexity of a routine that goes looking for the maximum power point does reduce this concern. but for 'emergency' operation, the last few percentage points of conversion efficiency aren't your main concern.