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I wonder if a 90-degree pentaprism might be a good option for extending the sextant's reach an additional 90 degrees (up to around 220 degrees if the base sextant is good to 130 degrees).  I have only read about them when used with theodolites, so I don't know the best way to mount one on a sextant or how to align it once mounted.  As Brad Morris points out, the angle needs to be known and stable---no problem with this class of prism.  Perhaps alignment could be done interactively by turning the prism slightly in its mount (to get everything coplanar) while measuring.