There's lumberthing I want to tell you

[glatt]

I would guess that there is not much heat in those radiator supply pipes. They probably get warm, but not hot to the touch..

I figure you have 3 options. Just box them in and don't worry about it. The warmth in the pipes would warm up the woodwork slightly and would contribute to the warmth of the overall house because they run inside the envelope of the insulated house. Or you could put in vents like you suggest to try to let the warmth out of the built-in unit and allow it to get into the house a little more quickly. Or finally, you could put some pipe insulation on those pipes to try to get the warmth up to the radiators and not "lose" any in the enclosed space. I put "lose" in quotes, because the warmth will still pass slowly into your house if you don't insulate the pipes. But by insulating you can try to focus the heating in the upstairs room with the radiator.

The only worry with pipes along an exterior wall is that they can freeze if it gets really cold out and your heat happens to go out. But those look like PEX pipes, and PEX pipes do well when they freeze.

If it were me, I would probably just buy a pack of pipe insulation and insulate the "hot" pipe to try to get that heat up to the radiator. And I wouldn't put vents in.

I'd be curious what you decide to do, but I don't think it matters much.