Yes, you can absolutely start pre-emptively. In fact, a good number of projects in the past have done so as part of the pitch to the publisher. Way back in the day, id Software (then called Apogee Games) famously [pitched a port of Super Mario Bros 3 to MS-DOS] to Nintendo. Nintendo was impressed but ultimately turned them down. The technology was swiftly repurposed into [Commander Keen: Invasion of the Vorticons].
However, it’s worth noting that any material made for a pitch practically never makes it all the way to the final product. Remember, a pitch is just a demo to get buy-in from the publisher. It does not need to be fully-featured, it does not need to be optimized, it does not need to be bug-free. It doesn’t even have to be reasonably playable - it just has to hold together long enough to demonstrate a solid idea of what the game could be to a roomful of executives for the few minutes it takes to go through it.
The purpose of making the assets with a licensed IP as part of a pitch is more to demonstrate the team’s commitment to the brand than it is to get a head start on production assets. If the publisher is looking to use that brand in a game, it is more persuasive to have a pitch tailored to what they are looking for. A large part of any pitch is giving enough direction to show where you’re aiming while leaving enough room for the viewer’s imagination to fill in the details. Making good use of a license pre-emptively can be a very strong way to secure that mental buy-in.
Got a burning question you want answered?