the path to the parts library gets saved in the user registry under (i.e. ST7):
HKEY_CURRENT_USER\Software\Unigraphics Solutions\Solid Edge\Version 107\Library in the key named PartsLibraryFolder
I didn't investigate so far how to get Solid Edge updated without restarting it.
Thanks Wolfgang. That was very close.
I tried with creating a new assembly but that does not seem to be the point in time when SE reads the registry for the Library path. Also as you said there is no way in sight to get SE updated without rebooting.
This was required in context of this macro where I resorted to use the 'Windows Explorer' to open the path intead of the Part Library of SE.