Why not use actual ghost chilies (maybe even just one)? You could just add them to secondary, I've done that with peppers before.
The oil-based extract is a bit of a concern because you don't know if it will get thoroughly emulsified and distributed into the beer. If you really want to use an oil-based extract, add something acidic to it (a little lemon juice). You could add it at the end of the boil to be sure it gets mixed in, or you could use a secondary where you put the oil in the bottom of the secondary and rack on top of it. You definitely want some good mixing action.
Some people will say adding oil in any capacity will kill the head retention, in my experience that's not really true (2lb of peanuts in 1gal = good head retention, for example... and hops are loaded with oils)... but an alcohol-based extract would be the better method if you want to use an extract.