I was discussing the Six-Pack with a friend recently, telling him I planned to do the remaining three this year. Though I knew Rose Peak was accessible from both the west (Sunol Park) and the east (Del Valle), he mentioned that the Sunol entrance is easier. I've done it in previous years from both directions, without really thinking about the differences.
It appears that Sunol is about 2 miles further, but about 1000 feet less climbing. (It avoids the infamous “Big Burn”, but also skips Murrietta Falls, which is very nice in the rainy season…dry at other times. “Goat Rock” is along the Sunol route, but it's technically off-limits.)
Yeah my route might not have been entirely legal. No signage in the direction I took the loop, and the map at the time wasn't very clear. I didn't realize until I closed the loop and had to cross a gate that my route might not have been legal. I'd stick to the high route as an in-and-out to be safe. Unless the Ohlone wilderness map has been updated to make things clearer.