The idea of a character acting as an Emissary of Power for an Outsider is an interesting one, for a story. It would be completely out of place in a DRRPG game in my opinion. Keep in mind the 7th Law, "Never Power and Knowledge from Beyond the Outer Gates," means that the White Council is going to deal with ANY mortal using Outsider-sponsored magic with extreme prejudice. Harry has even mentioned that too much knowledge of Outsiders, the Old Ones and the Outer Gates could be considered a 7th Law violation.
A (mortal) character with Sponsored Magic from an Outsider is an automatic violation, which would lead the nice, friendly neighborhood warden into paying a visit to give the character a haircut. At the neck.
In short, the concept is going to do either one of two things. 1. The Warden and/or other members of the White Council and their allies are going to be ordered to kill the character using Outsider magic, or 2. The entire group is going to be treated as warlocks or minions of warlocks, and dealt with accordingly by the White Council and allies.
While not very much is known about what Outsiders want, they seem to be hostile to life and reality as it is known to everyone in the world and Nevernever. In short, they are thought to want to unmake the universe. With that in mind, virtually every member of the supernatural community, except for the Circle/Black Council/Black Hats and maybe the White King
are going to want to stop/kill the group. Either because they want to keep the Outer Gates closed, or because they want to harness the power of the Outsider(s) for themselves.
Now if the campaign is planned on being very short, and the players understand beforehand that their characters are going to get killed, the concept might be workable for a period of time. For any sort of extended campaign, the concept is just too unworkable within the DFRPG universe.