I spent some time fooling with solar panels for charging reasons and what I found was that the angle of the panel to the sun has an enormous impact on the power output. I would guess that the power output is pretty much correlated to the strength of the sun that the panel is seeing.

But taking that for what's worth, then you might be able to use that as a figure of merit if the lux was constant with time (bad assumption) as to how much sun your skin is seeing - should be fairly accurate for the skin at your wrist since that is going to be oriented to the sun at the same angle as the solar panel in the watch. Unfortunately, if I understand Lux correctly and how sunburn works you'd pretty much have to integrate the lux number over time to come up with a (forgetting units for the time being) total photons or total energy that hits your skin. For example, if you were getting 300 lux for one second, you'd not get a burn. But if you did that for an hour, then probably much more likely.

Then with the lux number, I'm guessing that it's going to change radically as you move your wrist - again arguing for the need to integrate the data to get to an exposure over time number. Will it give you an "average lux" or something like that?