So, why not just use a primary battery and forget about solar altogether? Batteries are easier and cheaper, but for low latency remote control, you're either going to need big batteries or be prepared to change them a lot. With solar, I'm hoping it will be both smaller and set-and-forget.
For instance, if you want to turn on a light or a motor or something at the push of a button, your receiver will need to do a lot of receiving, and at 16ma that adds up. Even with listen mode, it adds up if you want quick response: say, within 100ms latency, or maybe 1 second tops. Button cells alone won't last very long in that application before they need changing.
So, if there appears to be some overkill in the design, it's because I'd like to also have it cover this more challenging use-case of low latency remote control plus maybe do more, like light the light or spin the motor or make sounds or what have you. I think the charger itself is already about as good as it's going to get, and so the variables I'm playing with now are just panel size, panel voltage, and capacitor capacity and self-discharge rates. Plus, any improvements on the moteino side reduce those requirements. If I do this, I really do want it to be robust and highly reliable, and so I'm leaning towards overkill rather than run the risk of underkill. Otherwise, it's back to batteries....