Reddit mentions of Alise 1-Inch Dia Shower Curtain Closet Rod Holder Pipe Flange Socket Ceiling Mount Bracket Pipe Fitting Parts Supports Can Adjust Fixed,FL5005-2P 2 Pcs Stainless Steel Chrome Finish

You need a curtain rod with long stanchions

See my little doodle for reference

Essentially, you find the longest curtain rod stanchion you can find and mount them directly over the window sills, instead of on either side like you would normally

Amazon Link to example of 6" stanchion

This gets the curtain in front of the pipe, but not covering the pipe or near it to cause a heat issue.

You could also drop the curtain down from the ceiling in the same manner...but finding a curtain stanchion sized perfectly for your needs might be difficult. In that instance, you might be best served getting galvanized pipe and fittings and attaching it through the ceiling drywall using toggle bolts. Something like this

Then just get a nice glossy spray paint and paint the pipe whatever color you want

But regardless, the curtain will either have to be inside the sill like a shower curtain (I'd go with 2" window blinds instead cus it will look better than an interior curtain) or...you bring the curtain out in front of the pipe. The downside to that being you will have light spill on the ends. But if you are just doing sheer curtains or semi sheer, it will still have the same effect