Allow an initializer to suppress an existing channel?

Would it be interesting to have a way for an initializer to suppress an existing channel (e.g. by returning channels: {y1: null}) ? Currently me can only merge (so add or replace), but not censor. 

Like #2135 this enhancement would incidentally help with the waffle tips (#2132), but is not necessary for them, and should be discussed on its own merits.