That depends upon your tolerance for roasty flavors. I'm no expert, but I think that 3/4 lb. of roasted barley would be pretty darn assertive in a 5 gallon batch, and it would probably overwhelm a poor 1-2 gallon batch--not to mention black patent on top of it. On the other hand, this recipe will make beer. If you don't mind taking a large risk of wasting these ingredients, try it out and let us know what you think. If it were me, I'd go pick up a few more lbs. of DME and make a larger batch of more normal stout. Have fun!
"Das Wort hat, ... als ich Wittenbergisch Bier mit meinem Philipp und Amsdorf getrunken hab, also viel getan..." - Martin Luther
On Deck: American IPA, Märzen
In Bottles: Prime Abbey Singel, Vespers Abbey Dubbel, Terce Abbey Tripel, Compline Abbey Quad, Dragon's Tail Barleywine, Boyne Mead