It does feel odd to me that there’s no FL content from backing whatsoever. The only reward over 30 pounds that I care about whatsoever is the star map, and I’m not paying 150 pounds for a single item that might be sold again later. I’m not even sure whether I will spend 30 pounds as a second copy of the game is useless for me.
If the issue is concerns over FL story rewards taking time away from Sunless Skies content, why would it have to be something as major as a story? I’d happily go from 30 to 40 pounds for the space equivalent of a Parabolan Panther item. As in, some equippable Sunless Skies-themed item appearing in both games, no story or additional content attached. I suspect that most of the time spent on the Parabolan Panther backer reward went to writing, coding, and testing the Panther’s story and the kitten mechanics, rather than writing and implementing the Parabolan Panther companion itself. (If Failbetter can implement three items and related storylets for each FL faction converted to Renown, a single item for a Kickstarter reward should be well within possibility.)
As for later arrivals missing out on backer rewards, isn’t that the point of Kickstarter? The entire model relies on backers coughing up money for a final product that doesn’t yet exist, may never exist in a form they want, and will cost less than most of the backer tiers. Giving backers something special to attract enough people is integral to the system, and often that something special is Kickstarter-exclusive. I’m disappointed that I missed the FL content for backing The Silver Tree and Sunless Sea, but I’m not angry it exists - I just hope it goes on sale eventually and enjoy the rest of the game in the meantime. Backer-exclusive FL content is barely different than all the free items no longer available, yet I don’t see people complaining about being denied Forbidden Map-Fragments or Primitive Hats or Archaeologist’s Hats or Marvellous Model Ships and so on.
If exclusive content is really that big a concern, why not make something where the exclusivity is time-limited? For example, let’s use my hypothetical 40 pound backer tier. Backers at that tier and above get a Space Telescope in FL, a Watchful +5 weapon: decent but nowhere near best in slot. Then after a set length of time - say, when Sunless Skies is officially released - anyone can purchase this Telescope for a higher price. (AKA higher than ten pounds, but I’m not sure how much Fate that would be in practice.) To keep the sense of backers getting something special, give them all a metaquality thanking them for the support*. Thus backers get something unique, and other players aren’t locked out forever.
*"I supported Victorian Space Imperialism and all I got was this lousy metaquality"
That’s essentially the same structure as Exceptional Stories have already. Anyone in the right place at the right time with the right degree of financial security has an incentive to spend money, and those who already would spend money get something in return. Latecomers or those who didn’t spend money can still get the reward, if they’re willing to wait for it to be available and then spend a higher amount. While I’ve seen occasional complaints about the ES model, the objections weren’t about the players supporting FBG financially getting a reward with a time-based incentive. (I think the complaints were mostly concerns over a monthly story taking time away from free FL content, which wouldn’t apply to a one-time small reward.)
So yeah. Myself and assorted others would definitely be interested in a FL backer reward, and would care enough to give more money (or to give in the first place). The delayed-availability ES model is a compromise between "backers get something special" and "non-backers are locked out", and I bet there’s other models I haven’t thought of. A simple reward wouldn’t have a big time cost, so why not offer one when it would attract more backers and a higher chance for the Kickstarter to succeed?