When using another mechanism to transfer the data like Data Domin the Aux copy will perform a Lan free aux copy. all you have to do is pick the media agent on the replica side with access to the replica library. The software will know to use the replica size for the data. it would be the job of the DD to make sure the data gets replicated.
This is only true if you have a replicated mag lib setup in CV. Wether we are doing the replication or not does not matter, what matters is how you have the library and its paths created. We will always default to the replica MA for aux and restore if the library\MA being used is on that side.
To the original question, yes this is possible and is the recommedned way to do it for the reason you quoted, the aux will go over the link from primary otherwise. This has nothing to do with setting the copy to come from primary or anywhere else as the source the primary is still what we want its just the replica side.
Requires a mag lib setup with the primary and replica path and again it has no bearing on if we do the replication or if it is done outside CV.
Please let me know if you have further questions on this.