I've never heard of anybody using dry fruit (except raisins in JAOs), but there's not reason it wouldn't work; the sugar is still there. I don't think you'd have to rehydrate them before use; just chop em up and throw em in.
From what I remember, dates and figs are pretty subtle fruits to begin with, so it may be hard to get their flavor coming through strong in a mead. If I were personally making the mead, I'd add more along the lines of (assuming the fruit is pitted):
Firstly, you probably want the final product a little less alcoholic, because the alcohol might mask the flavor of the fruit a bit. Also, since you're working with dry fruit, and these two fruits especially, it's pretty much all sugar, so your suggested recipe is probably equivalent to around 4lbs of honey per gallon.
Also, you may want to wait until secondary to add any spice.