I've used invert sugar in a chocolate stout before and it worked great as a gravity booster. It doesn't affect flavor very much and you can use a small amount to good effect. Lyle's Golden Syrup is a good example of this. Treacle, molasses, or honey can be used too, but these will have some influence on the flavor (which may be desirable!).
If you're going with DME, follow the calculations on William's Brewing, Home Brewing Supplies, Winemaking Supplies, and Home Coffee Roasting. @ Williams Brewing
(scroll down to 4.1) and use the lightest DME you can find. A stout - especially one such as a chocolate raspberry stout - will be forgiving of other minor flavors and may hide them entirely.
As for the oats... use something flaked for best results, like flaked barley. I've never seen more than a pound used in steeping with extract either.
But then again, it's homebrewing. Go crazy. Experiment. Don't be afraid to try something new! If you're not careful you might invent a truly amazing beer!