E.g. instead of showing a central component MyApp in the center, I explicitly show MyApp-iOS, MyApp-Web, MyApp-Backend etc?

Short answer

No, you shouldn’t do that.

Longer answer

Although you could quite easily combine context and building-block-level-1 and then start the actual building blocks-view with level-2 - you would dilute the context and building block view…

Actually, building-block level-1 already contains both internal structure of your system AND external systems - so instead of repeating the systems’ internal structure in the context you should clearly separate the scoping (context view) and the internal building block structure.

Exceptions allowed?

Actually I (Gernot) experienced such a situation in a fairly large system that included both embedded, graphical-client AND server-backend software (among others). We used the context view to depict the top-level subsystems together with the external interfaces…

Due to the heterogenous nature of the top-level subsystems we created several different “low-level” arc42 documentations, one for each of these subsystems.