because they just hijack the signal to the display and route actual boost data item to it
I am not familiar with car software specifics, but I highly suspect those labels are hard coded into the headunit software/firmware or whatever, making it very hard to modify as the code running the headunit probably needs to be signed. The data values displayed come in via inputs which can be hijacked and manipulated/replaced on the bus.
Fun fact, if you long hold the reset miles traveled button on the bottom left corner of your dash where mph and revs are displayed. You'll be taken to a screen in which you'll find a few extra features, usually accessed for dynoing amongst other things. However if you single click the same button to scroll down you'll find an "unlock" prompt. Long hold again to access it and you'll be prompted to input a value. This value should be the last 6 digits of your vin added together. Single click again to go through the numbers starting from 0 up to whatever you added. After you input your added value, long hold again and it'll back you out to where the unlock prompt you clicked on was. However, this time if you scroll down, again by single clicking, you'll find a temperature prompt, which when long held, will bring up your coolant and oil temps. Downside is that you'll need to repeat this process each time you restart the car.
I don't think they will allow us to create any. My understanding is that they may allow car companies to customize things to their liking however as an incentive to jump on board. While I'm not sure how it will play out, some think that car companies may not opt to install Car Play 2 because it means giving up a lot of control and monetization potential. Customization is Apple's polite reach around. ?
