Maybe rather than a set number of abilities the houngan is given during the possession, he could have Modular Abilities, which would let him have different abilities depending on the loa/orisha/orixa possessing the captor.

I'd also go in for full-on Thaumaturgy, since he'll need to be able to do curses as Deadmanwalking pointed out and he'll need to be able to put together gris-gris bags (AKA mojo bags).

Considering I'm going to college in Miami, I'm trying to put together a group so I can Dresdenify it. I've got some decent ideas for how to supernaturally spice up the city so far, including:

• The White Court having a controlling interest in South Beach, living off both the despair of the drug trade and the lust generated by the clubs
• Merfolk, the spawn of humans and aquatic denizens of the Nevernever, who take on characteristics of the sea creatures they spend their time with
• The Loa playing a major role in the local spiritual politics, maybe with somebody like Baron Samedi taking himself a "horse" and setting up his own little fiefdom
• The Summer Court having the upper-hand and taking a particular interest in students at the University
• The Winston, a supernaturally-charged dive bar based on my favorite watering hole, complete with a half-ogre doorman and a storyteller druid

I'd like to see Lucius Glau (the jann lawyer from Proven Guilty) statted up. I've always liked djinn and genies and such, so I was kind of annoyed that the only one in the series to date was such a wimp. I've got an idea for a jann character, but I wondered how everyone else viewed jann.

Another thing to remember is that if you go back far enough, wizards would be able to keep current with technology until about the 1950s.