From what I understand: SetString(...) is indeed the path to go.
I would say: if it works some times and fails at other times: is your path an absolute one?
I mean: on windows platforms, an absolute path starts by something like C:\... or a net path, such as \\my_server\my_folder\...
On Mac platforms, if I'm not wrong, an absolute path starts by "/".
If you edit an MT5 file or an MC6 file, using a simple text editor, you should be able to retrieve how Poser stores that absolute path.

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