Since the Subscription.channel.type is a code with a required binding, we cannot directly use this field for additional channel types.  That said, general agreement is that the functionality is important to include in the specification.   In that light, the recommendation is to define a modifier extension on Subscription.channel.type (e.g., backport-additional-channel-type : Coding) which can be used to include custom channels.  When present, this value would override whichever value was present in Subscription.channel.type.  This extension is unnecessary in R5, as Subscription.channelType is a Coding with extensible binding.  The values from this extension should be directly mappable into that structure.