They wouldn't need to vet everything, only the best of the best.
Make the "template files" they already have internally for creating station parts exteriors/interiors available to the public. For the interiors they probably have one for each slot size: small/medium/large. For the exterior there are probably "attachment points" on each end of the Orbis segments that follows a standard so that each part always will fit with the next one logically. Also put out a design bible when it comes to the style of the different factions and general look of the game. Also things like polycount, size and other limitations would be made clear in the documentation. These things already exist...otherwise they wouldn't be able to work on it anyway.
Then create a section in the forum where people can publish their creations. Every month or so people can vote on their favorites. The top 5 submissions can then be "vetted" by FD. Only if they find that these live up to their expectations they will move forward on them. They can then just check if they have been built to specs...if they haven't they simply give feedback and let the original creator fix it. When fixed they put it into the game. People who put their work into the game also need to sign a form where it's made clear that they from this point on will no longer own that asset. They WILL however be mentioned in the credits of the game. Always something to put on the CV. Who knows...if someone turns out to be exceptionally good over a long time FD might even hire them!
As I pointed out in my "Copy&Paste" above this would only apply to static "window dressing" in the game world and not dynamic objects like ships, weapons or people. Modules on the exteriors of stations is for example not something you will interact with in any other way than looking at it or, at most, crash in to.
Further down the line this could also be applied to variants of citybuildings, plants, potentially alien body parts (think Spore) and so on...
This way the FD devs could focus their artists on putting together the assets we will dynamically interact with (ships, planetary vehicles, weapons, equipment...) while the "crowd" could create the "windowdressing".