I am looking for an online registry that the owner of an authentic
World War II Half Track military vehicle could use to bring the
availability of his half track (for a fee) to the attention of movie
makers.

This heavily depends on where you are, since I doubt you'd want to
take your half-track to just anywhere in the world. The Hollywood
Creative Directory publishes the widely used "Blu-Book Production
Directory" for Southern California, so I'd assume their online service
is eually comprehensive - http://www.hcdonline.com/

You might be better off getting in touch with a company that rents out
historic military vehicles for films, and try to cut a deal with them.

Thank you, Lesteresmore. I appreciate your help.
The owner in question told me he believes there is an online database
based in New York City. I'll have to get more information from him
before proceeding any further on this.

Thanks again.

Wawa734,
Just wanted to know if you've found what you're looking for. Perhaps
you've not found any leads yet, and you might want a researcher here
to try looking for them.