Hey Zman -
I would go with the oats over the maltodextrin. I use them in a lot of my stouts with good results. I have never had a sparge problem with oats in the percentage of total grainbill that you have (10% +/-), so I wouldnt worry about the rice hulls.
Now, as for the 120 min mash - I (personally) wouldn't go that long. With the FG in the 1.018 range, you can expect a fuller body more than a dry style. So, there is no point in making a more attenuated wort. The 156 mash temp also lends itself to a slightly higher FG. I have never gone over 90 min. for a single temp rest mash. FWIW, I mash nearly all of my stouts for 60 min.
That said..... if you have gotten the recipe from a tried and true source....maybe it is the way to go!
You know the saying about opinions....!