Display, storage space can transform unsightly wall
In looking at your sketch, the first thing that strikes me is how unbalanced the wall is with the window; so off-centered.
The second major problem is the radiator and not wanting to enclose it. The most obvious and easiest way to camouflage many unsightly radiators is to cover them with grill work or louvers. This will work in many instances, but not in your case, because any type of enclosure will cut down on the heat to some extent.
Because the room is a den/study, I’ll assume that you might need additional display and storage. With that in mind, there are a few different things you could effectively do to visually change the wall.
I would balance the window with shelves to the far left side of the wall, approximating in width and height the size of the window.
Across the entire 10-foot wall at the ceiling line, I would install a wooden valance approximately 8 inches from the wall and 12 inches deep. This will unify the wall space and make the room appear wider.
ABOVE the shelves on the left and the window on the right, I would install two mini blinds which would be mounted under the valance. These will not only cover the window, but will not block any light when raised or tilted. They will also help to balance the wall and conceal things stored on the shelves. The effect will be attractive as well as functional.
Pleated shades could also be used, but draperies would be too heavy and would block part of the window and shelving when open.
There will now be approximately 48 inches of open wall in the center above the radiator. You could mirror it, or hang a piece of art or your collection of family photographs there.
Think about installing a strip light on the ceiling under the valance. Mount it as far as possible away from the wall to help prevent shadows. The light will highlight whatever you choose to put there.
You could also install additional shelves in this 4-foot space for display or more decorative items than would be in the readily covered shelves to the left.
If you install the light under the valance, glass shelves would be attractive with the light filtering down thru the various levels. If you’ll need every inch of space and the shelves will be filled with books and other.
If the shelf is deep, approximately 24 inches, there could be a desk on the left. Desk supplies could conveniently be stored on the shelves behind the mini blind.
There could be additional open shelving under the window on the right. It would not only provide more storage, but the shelf would be an ideal spot for plants.
The two areas of shelves and open spaces of the window and knee hole would asymmetrically balance each other. It would be an interesting, but simple design.
Another mini blind could hang beneath the shelf. It would cover the radiator in the summer and be raised when the heat is needed. Bi-fold doors could also be used to serve a similar function, but again, the mini blinds would counter balance the others. The effect would be aesthetic and inexpensive as well as practical.
If you don’t need a desk, or the room isn’t large enough to accommodate the depth, the shelf could be much narrower. Make it slightly deeper than the radiator and put shelves or enclosed storage on either side. Camouflage the radiator in the same way as if you had a desk.
Regardless of what you choose, the effect will be dramatic and functional. More importantly, the wall will look as though you planned it that way.
As is often the case in home design, what is originally perceived as a disadvantage becomes an advantage.