I am used to playing Dune with the following house rules:
(The reasoning is that this is the special power of the Emperor and so it should not ordinarily be allowed. It is true that you may be able to accomplish the same thing by use of a third-party non-ally intermediary, but on the other hand you may have to pay that party for his or her services, or that party may fail to deliver on the agreement. It seems to make for a more interesting game.)