I'm not sure that's a good goal. We already have a mechanism for doing this e.g. the replaceSubrange method (though we don't have such a method on non-RRC collections, but could). Why is it important to facilitate that operation through subscript assignment?
The purpose of get/set properties and subscripts is to allow them to behave to the user like l-values that can be mutated "in-place"*, as if you were mutating a regular stored property. This means you can, for example, write swap(&a[i], &b[j]).
But what's proposed is not doing that. It's just providing a new way to write mutating methods using subscript syntax. I agree with Dave that this weakens the meaning of subscripts.
*in use, if not in practice, pending language enhancements and elusive solutions)