Personally, I doubt its a west coast super mutant, just for the simple fact Bethesda has largely left to the west alone, on purpose. Outside of bringing some of the BoS east, and a few references to Fo1/2/Tactics, Bethesda has largely left everything in the west/midwest in their own bubbles basically.
Avellone said during an interview from the time of NV that Bethesda gave them the entire western U.S. to work worth, which suggests Bethesda is just fine with leaving the west to Obsidian, since they are the guys who made the western U.S. lore in the first place.
And in The Pitt DLC, Ashur and the slaves talk up places like Erie, Ronto, D.C., and The Commonwealth, all of which are north/south/east of The Pitt, but no one mentions anything to the west, even though Pittsburgh sits right near the border of Pennsylvania and Ohio. Which is odd given that the Pittsburgh -> Chicago area is home to the largest concentration of American metropolitan areas, including Cleveland, Columbus, Cincinnati, Detroit, Louisville, Indianapolis, and OFC Chicago, many of which are closer to Pittsburgh then Ronto is, several of which are closer then D.C. is, and all of which are closer then Boston is, by road travel.
Its very odd, and basically impossible, that NOTHING of significance would rise out of such a massive cluster of metropolitan areas in 200 years time, which suggests Bethesda intentionally avoid the Midwest on purpose, likely so they didn't have to address the MWBoS elephant in the room.
If Bethesda has gone so far as to basically just give Obsidian the west, avoid mentioning the Midwest as much as possible, and even creating their own strain of super mutants, just so they wouldn't have to use the west coast ones, its very unlikely they would pull a 180 and start throwing in west coast Supers Mutants now.