alt Hacker NewsAh, I didn't read their comment too strictly.
UVB synthesizes cholecalciferol (vitamin D3) in the skin, which the liver converts into calcifediol (what blood tests usually measure), which the kidneys convert to calcitriol (the active hormone). Wiki claims the kidneys have a negative feedback loop, converting excess calcifediol into inactive 24,25-dihydroxycholecalciferol. I wish I had better sources (for my vitamin D pdf folder).
https://en.wikipedia.org/wiki/Vitamin_D#Excess
But just knowing that, I don't immediately see anything limiting cholecalciferol or calcifediol amount and storage.
The same wiki article says there is a limit to the capacity of synthesis by UVB due to the quantity of reagent 7-dehydrocholesterol produced in the skin, but I don't know the math on what amount of exposure would be required to hit that limit - presumably it (or something like it) is covered in the article above.