The proposed ways for actors to opt in to custom executors are brittle, in the sense that a typo or some similar error could accidentally leave the actor using the default executor. This could be fully mitigated by requiring actors to explicitly opt in to using the default executor; however, that would be an unacceptable burden on the common case. Short of that, it would be possible to have a modifier that marks a declaration as having special significance, and then complain if the compiler doesn't recognize that significance. However, there are a number of existing features that use a name-sensitive design like this, such as dynamic member lookup (SE-0195). A "special significance" modifier should be designed and considered more holistically.
There was a pitch about this, a few months ago: Pre-Pitch: Explicit protocol fulfilment with the 'conformance' keyword