What you say makes sense and yes, Mongo adapters do use a custom converter, but that comes from spring data mongo and is used to instantiate the MongoTemplate if one is not provided.
I am comparing this more to the Redis's ***o.s.i.r.oRedisStoreWritingMessageHandler*** as that would be a better comparison.
Merging in a way you have implemented shouldn't be a issue in my opinion as we can anytime change things in future to accommodate the changes.But do take a look at the way Redis's adapters are implemented and let us know your views.