I am definitely planning to vent it somehow. I haven't figured out the best way to do that. The simplest way is to just make the back of the bottom shelf open, so air can move around, and to be careful of what I put on that shelf. Another is to raise it up on legs.
One of my major goals for this project is to make this maintainable. I care about functionality much more than aesthetics. Many of the traditional radiator covers are dust magnets, and I'm trying to come up with a design that's easy to clean. More broadly, I'm planning to cover the bookcases in some way. A barrister bookcase would be awesome, but I think too much time and material. I'm thinking of either curtains or sliding doors (there isn't room for doors which open out everywhere).
More broadly, to give an idea of the space, it's a bit over 7 feet high. It's 24-feet long, but there's a structural member in the middle, so it's naturally two 12-foot spaces. Each of these can be further subdivided into three roughly 4-foot segments, each with its own shelves and cut-outs (we have two windows and a wall-mounted AC which naturally subdivide-up the space).