Yes, you're right that Rose Peak is the highest attainable point in Alameda County by the public. However, aside from Mt Hamilton which is really not open to the public for hiking, there is nothing else publicly accessible that is higher than Rose Peak in the Altamont Pass-to-Pacheco Pass block. Henry Coe is much lower and so is Mission Peak.
Maggie's Half acre is named after Maggie Rowell who leased a half acre on the peak to the USGS for the installation of seismographic instruments.
Mt. Rose isn't actually the highpoint of Alameda County. There are at least two other higher points, such as this one dubbed "Discovery Peak" back in 1991 by the person who found this in perusing the topo maps. Since it sits on private ranchland, Mt. Rose can accurately be called the highest attainable highpoint.
Also, there are other accessible peaks between Mt. Diablo and Pacheco Pass in the Diablo Range. Mt. Hamilton, of course, at over 4,000ft is the highpoint of Santa Clara county, and one can drive there. There are also other peaks, some very remote, in Henry Coe State Park. Mt. Sizer at 3216ft is fairly easy to reach from the Main HQ of the park. Mission Peak and Monument Peak also fall in the Diablo Range and are accessible to the public.
Maggie's half acre is the best campsite if you're going to the peak. It is just to the North of the peak and is halfway between Del Valle and Sunol. There are several sites here with water. I stayed at Doe Canyon horse Camp one year and it is a terrible campsite.
Daily parking fees @ Del Valle as of 4.22.'5 are $6- bullshit. For an additional $2, however, they give you a nice regional map.