Thanks adp001/ironsoul, those all sound reasonable.

Calling poser.TempLocation() fromPP2014 on my Windows 10 system returns 'C:Users:USERNAME:AppData:Local:TempPoser Pro:10'

...which is a few levels down from %APPDATA% (I guess 'C:Users:USERNAME:AppData)

...which is itself I guess one level below %USERPROFILE, 'C:Users:USERNAME

And I've now found the EZSkin files. They're in the same area, but under 'Roaming' not 'Local',e.g. C:Users:USERNAME:AppData:Roaming:Poser Pro:10

SoI guess that the original query "How do you avoid UAC problems writing to poser.AppLocation() subfolders ?" is best answered by "Don't try to write there !"

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The 3Dcheapskate* 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 or something else.
*soon only to be available in two flavours -  DAZ and Renderosity