Watch Face trial mode

Hello guys! 

The developer’s guide mentions the possibility of making the application paid by using the functions of isTrial and two more. But it is also said that they cannot be used with watchfaces.

But how so? For example, a watchface called Date Lover has a DEMO mode, which is unlocked using a code sent by the developer.

Are there any real ways to add DEMO mode to the watchface? Thank!