Are we
all in favour of the idea then

I did say that it was "unlikely to ever take place".
Personally I'd love it if offical packs where released for Star Trek (different series), Star Wars, Babylon 5 and Battle Star Gallatica for GalCiv2 and other games.
I'd buy them all and have loads of fun.
So to recap - a company would do/hold the ships/stations/models for the participating brands. Whenever a game was released and proved popular, the company would speak to the game developer and brand owners.
Some brands would agree an add on pack, others may not (it's their choice) or may choose to wait longer. All brands would be told what the others are doing, and scheduled release dates. The company then provides any necessary data to the games developers in a suitable format.
The games developer then creates the add on pack. The inbetween company meets with the brand owners (after doing its own user testing) to demonstrate it - and the pack is released online (both from the developers site, online retailers like amazon - who track purchases made - and optionally on the brands official website). I don't know how worthwhile it would be to put it in retail outlets.
Revenue would be split between the brand owner, the game developer and (unless bought direct) online retailer. The inbetween company would also take a share - which would vary depending on whether it's a normal company or if it was created/owned by the brand owners.
Owners of the game would purchase the add on pack of their favourite brand(s). The value added part for them is playing ships/stations/characters from their favourite tv shows/movies.
We know that a market does exist for this at least to a small degree - just look at the mods for this and other games. Mods however are difficult for the average person to use, usually limited in scope, take ages to make and are not to the same overall quality.
Companies that have paid money to develop branded games for x number of years could also work with it. Star Wars lego isn't likely to be affected by an add on Star Wars pack for Gal Civ 2. But the game developer and/or brand owner could get more revenue for allowing such a pack to go ahead. And if they produce games like Star Trek Bridge Commander - these developers and/or the brand owners could get more money and more sales by allowing other brands to do an add on conversion.
But I fully agree that its never going to happen