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The only viable option as I see it, is to export the bent geometry, morph it, load the morph into the figure, then apply another morph to unbend it. That's what I tried doing, but I obviously did something wrong.

No, that simply can't work. To flatten an unbent thigh, you need to move vertices in the z-direction. To flatten a thigh bent 90 degrees, you have to move vertices in the y-direction. But adding or subtracting another morph does not change the direction your vertices move in in your flattening morph.

ETA: colorcurvature made a tool called PoserMorphLoader for exactly this kind of thing, but it seems to have disappeared from the MarketPlace. You may have to contact him. The only other tool I know of that can do this is MorphLoaderPro for DAZ Studio. Otherwise, you'll have to do the morph on the unbent figure and hope that it looks correct after the bend. Or maybe you could convince Poser to apply the morph after the bend, which should also solve your problem. I seem to remember that that's how the morph brush is supposed to work, but I've never tried it myself.

-- I'm not mad at you, just Westphalian.