Hi
The RLI Association can be found at http://www.therli.com assuming you're not already aware. I personally don't know of this specific documentary, but there are contact details for the various sub-branches within the Contact Us tab of the association's web page, and someone may know of the work you speak of. There are also a few DVD's out there that have been collated and produced in the last few years, and it may be that the footage can be found in one of them? Try http://www.rhodesianvideos.mazoe.com/

Wish you well in your search, let us know if/where you find it, a few folks here would be interested.

Cheers
I.R.