@Bjoern's fix worked perfectly with one exception, there needs to be a few extra lines at the top of the library.txt file. It should read, given the rest of @Bjoern, like this:
A
800
LIBRARY
# remove default tugs.
EXPORT lib/airport/Ramp_Equipment/Tug660_Dn_1.obj empty.obj
EXPORT lib/airport/Ramp_Equipment/Tug660_Up_1.obj empty.obj
EXPORT lib/airport/vehicles/pushback/tug.obj empty.obj
EXPORT lib/airport/vehicles/pushback/tug_driver.obj empty.obj
The comment is optional but seemed appropriate.
Thanks @Bjoern.
I have not thoroughly tested this but will post a correction after a dozen flights or so.