That's the one thing that Mactan lacks. It's a brilliant tool, but sadly, if you want to know which factions can use X faction's ship, you have to manually go through the list of every single faction to find out which ones can use, in this case, Core ships.
Maybe another page could be made where you simply select the ship you want, and it displays which factions can utilize it? I'm not sure how difficult to code that would be, though.