Minecarts are buyable in the Journeyman's Focus, and I don't want to see minecarts being used this way because then their value for use in, say, regular non-lava-usage minecarts would increase substantially.
Though, there could possibly be a Diamond Minecart, (craftable with 5 iron and a diamond via custom crafting, or maybe even buyable via cookies). It would actually be a spawn egg (renamed) that spawns a boat riding a diamond block piston entity template riding a minecart, with multiple invisible entities with moderate health in between.The diamond block will make it look diamond-y. I suspect using more invisible entities would be able to make the player invulnerable to lava while in this contraption, at least for a bit of time when the invisible entities riding each other started dying (acting as somewhat of a timer). With this, it would be possible to add this feature via just a button with command blocks (taking away cookies and giving them the custom spawn egg via a dispenser that gets refilled, or maybe even purely command blocks that don't require refilling that spawn (/spawnmob?) a chest minecart with one special spawn egg inside it in a specific location and then use hopper minecarts and hoppers to bring it to the player).
Using this idea, it would be possible to implement this purely in-game without any modification to any plugins.