The 'setting Pitch to a non-zero value and then setting it back' that I mentioned in the previous post - I haven't been able to repeat it. Setting Pitch to a couple of degrees down (e.g. -5°) does appear to get rid of the 'right half of the west-facing render groundplane'. But that's no help, since I need an exact 0° pitch.

However, following SamTherapy's suggestion I tried using the Poser 9 "Square Hi-Res" primitive prop for the ground instead. Even scaled at 10000% it seemed to render fine (I'd also raised the dollycam to be 72" above the origin)

So I now have my first 8192x4096 lat/long  (±180° azimuth / ±90° elevation) Poser-generated environment map (I used HDRShop v1 to convert from vertical cross cubemap to lat/long map). Here's a reduced version of my first success, with complete groundplane - no holes!

My attempts to understand why the 'right-half of the west-facing render' problem occurs are now floundering. But since SamTherapy's workaround seems to work for my particular situation I don't think I'll bother investigating further - unless somebody has a bright idea.

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The 3Dcheapskate (also available in DAZ and HiveWire3D flavours) occasionally posts sensible stuff. Usually by accident.
And it usually uses Poser 11, with units set to inches. Except when it's using Poser 6 or PP2014, or when its units are set to PNU.