You could fit a long curved track in there and have a bend in the corner subject to the curve being shallow.
That would allow a pair of curtains to swing around.
You can get black out blinds that run in channels either side that minimise light leakage to virtually zero but you need to take in to account the depth of the cassette.
In the above the red rectangles are the cassette blinds. The arrow shows you the void that the blinds can't run in to. Depending on the depth of the cassette, you may have nothing to fit the channels to in those areas.
With regards to curtains, you could use 3 separate poles as close to the windows as possible. Two curtains on the longest stretch and single curtains on the returns. The downside is that the curtains, when open, will block out up to half of each of the opening side windows. As there are 3 sections, I think cheapish tracks might be ok.
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