I think Pearl did indeed use stock footage from TTT. IIRC, They took advantage of the fact Richard Anderson had also appeared in the feature film. Whether it was unused footage I have no idea.
This might be getting a bit obscure, but FX footage from Space: 1999 was sold by a company specializing in library/stock footage, and the occasional shot not used in any episode was offered. These ended up in other tv shows, films, and even music videos.
Universal infamously pimped out Battlestar Galactica FX footage. The low budget film Space Mutiny as seen on MST3K is a good example, but I’m not sure if any unused footage actually appears.