You can view the NDMP Host ID in the "SnapVault" tab of the OSSV Configurator (assuming a Windows based systems) or using the svhostid executable in the install path (C:\Program Files\NetApp\snapvault\bin>svhostid.exe in Windows).
You are correct - the id produced by that command changes each time and I am unable to find any support documentation for it. I missed that it was changing when I ran it multiple times yesterday, however I have verified that it did not change the ID viewable in the OSSV Configurator, which remains the original value on my system.
Indicates a unique identifier that the Open Systems SnapVault agent fills automatically. This identifier is used by NDMP-based management applications to identify the primary storage system. You must not modify this field.
So if these hosts do not yet have working SnapVault relationships I recommend reinstalling OSSV on them to ensure unique host ids.