You could generate chemically costly preproducts for the plant synthetically too to help its efficiency. Things like increasing CO2 and providing at least nitrogen.

But while you are at it, you could directly to synthetic food... Hydrogen, methane, formaldehyde, sugars.

Sounds like the best solution mid term, before we'll be uploaded to computers. :)

I wonder if it's really necessary to convert the light to electricity as an intermediate step. I'm imagining a filter that's mostly transparent but has a fluorescent coating to convert UV to visible light that the plants can use, effectively brightening the sun in the useful part of the spectrum. Unfortunately Google is giving me the wrong kind of results for "fluorescent filter".

The material you are looking for is called a scintillator. It converts higher energy photons to visible. Of course it has its own issues / cost. Plus, you have to get the light from the rooftop to plants.