They would only do this if they put in a microtransaction model with intent of selling the content as premium.
This would be the business model:
1. The developers, artists, sound guy, etc would have to spend a few weeks just to make the ship model, interior cockpit model, textures, dev hookup of hardpoint, engine glowy bits and all the important details, gameplay balance and handling, sound effects, lighting, collision models, and hopefully lots of testing, etc. (Because this takes several weeks to build from scratch (and maybe even months) it will cost a bit of resources (to pay the people working on this new ship.)
2. To gain back the costs plus profit, this ship would idealy be sold for some new type of "premium currency" (which would also have to be implemented.) But for this proposal/example, I will assume that Frontier would go old school and sell this rare ship for regular credits (which unfortunately eats into your profits because the players who do not buy your currency (again, through the premise of microtransactions as proposed above) are not paying you, the developer, any money. And it costs real world money to build new content and features.
3. Set up a limited production run. I would not advise the "only 400 made" route (unless you are prepared to lie to the customer and fudge the numbers during the rare release period. What I *would* recommend is a limited time sale. This rare ship only available this week only. When the time is up, discontinue the sale.
4. Watch your metrics closely. The financial wizards of smart will crunch their numbers, count their beans, and see if the developer made their money back yet from the investment of building the rare ship to begin with. Whether in the black or the red, time marches on, and then you plan for the next inevitable phase:
5. Rare ship re-release, for a limited time only! (This is why we have seasonal candy, eggnog, mcribs... I think I'm making myself hungry.) (Is the mcrib an America only thing? I bet it is.) Why would you re-release a ship that was rare? Wouldnt it make it less rare? The answer: it doesn't matter, it makes money, and making money is all that matters. (Again, assuming there are microtransactions to buy currency.) If you don't make money, you can't pay your developers. If you don't have developers, you can't make new content for the game.
Again, this would only work with a business model attached to it. Making "exclusive" and "rare" ships is great fun for the end user, sure, but because so much money and work and time is dumped intk creating said ship, not having a method to offset the costs and turn it into profit is just financial foolishness.
tl;dr - Sure, I think rare ships would be fun. But let the ones with fat wallets pay for it because as we all know, "Time is money, friend."