> In my imagination, the lab would have some a testing process that spreads a precisely-controlled volume over a standard surface area, textured to be similar to skin, then measures UV transmission percentage vs wavelength with a diffraction grating and photocell. Or something like that!

With this approach, how would you measure the effectiveness of the sunscreen when it's been absorbed by the skin (which is necessary for the sunscreen to work properly - that's why they always say to wait ten minutes after applying before going out into the sun)?

There's a reason in vitro and in vivo are both studied for clinical trials of medications. Sunscreen isn't any different: you're using a product making a specific claim about a clinical outcome, so that needs to be tested.