The problem with creating fake Hipparcos numbers is that at some point, some developer somewhere will create an addon which conflicts with an existing addon in terms of the fake Hipparcos number used for a star system.
Now sometimes it is convenient to specify the Hipparcos number exactly, e.g. for specifying the Tycho catalog number of BD-10 3166, but at other times you just want any number, as the star doesn't exist in the Hipparcos catalog.
What I suggest is the use of a special number, e.g. -1, to tell Celestia to use the first available "free" Hipparcos number.
I realise there could be implementation of this, e.g. if an addon explicitly specifies a Hipparcos number that has been allocated to a star, but perhaps this could be resolved by a "two pass" approach, first scanning for stars with explicitly referenced Hipparcos numbers, and a second time to allocate numbers.