I'd be careful with button heads - the head is usually larger than a hex cap screw head. The cap screw heads are almost the exact same size as the McGard heads. They take up just about the entire hole in the center cap which leads me to believe the button head may not fit. The cap screws are also just under flush with the center cap so I'm not sure what a button head is buying you except less engagement with the hex wrench \ socket.
Buttonheads give you more friction surface between the bolt and the wheel. I've used buttonheads to replace the cover bolts, on other wheels. I didn't realize the Sy Ty wheel covers had such a small recess, i guess that i should have looked closer at one.