It would be a VPN feature similar to what Opera browser offers. One that turns off once the browser is closed.
The feature would be able to be turned on inside the browser to where whenever they start the browser the VPN turns on. It will basically be your version of Apple’s Private Relay.
You could look for a third party VPN provider to make their VPN work only in your browser. The appeal could be seen as advertisement for the VPN and it could get people to buy the VPN if they want more than just inside the browser.
My suggestion is Mullvad since it doesn’t collect any user data. They also might be willing to allow you to use their VPN because they need more people trying their VPN. There is also Orbot VPN which is free.