They would be using a pressure differential to transfer the fuel, not relying on just ullage thrust.

You'd be surprised at how low of an acceleration is required for propellant settling and transfer. On the order of 0.0001-0.01m/s^2. At a 310s Isp for the ullage thruster, that's between 0.01kg and 1kg of propellant usage per second. Even if it takes an hour to transfer fuel (on the ground, fuel is transfered in about 20 minutes and in flight, all the propellant is transfered from the tanks to the engines in ~8 minutes), we're only talking about maybe 1% of the propellant needing to be used for ullage thrust and perhaps as low as 0.01%.

ULA has published a paper on the topic: https://www.ulalaunch.com/docs/default-source/extended-durat...

They would build a dedicated cargo section and fairing for this, all of that would be fairly straightforward to accommodate. I don’t know if SpaceX has acknowledged the idea but there was discussion of installing ’retro-rockets’ up in the fairing for the moon lander (to reduce exhaust blast on the regolith), could use the exact same technology.

If they were so inclined they cold even recover the helium from the tanks they are filling and use that to assist with pumping pressure.

Wouldn't the thrust just be to settle the fuel so pumps could move it over?

Usually when you ignite an engine on orbit a short kick of ullage trusters is enough. It is because once the fuel settled enough to start the engine the trust of the engine will keep the fuel settled. This is not true in the fuel transfer situation, there a single kick is not enough. If you are using ullage motors and not internal bladders, you have to keep producing the micro-trust for the whole transfer duration. Be mindfull that pumping a rocket-full of fuel won’t be an instantenous process.

You need acceleration to settle the fuel and to keep it there. Once the acceleration stops, the fuel begins to unsettle.