Very interesting, ScottyT. Thanks for the link. This option may not be dead yet. I have to agree with Sailing Dog though... if it were that easy, everyone would be doing it. and while a small tributary in Annapolis might be very reasonable, out in the open Potomac is another story.
I need to find a rich friend with a house on the river... and take him sailing!
Using an anchor to secure your boat long-term when you are not aboard, would be pretty risky. A true mooring would be a much better arrangement.
And cement blocks don't make the best mooring anchors -- they need to be quite large, larger than you could manage on your own.
I think there are multiple reasons you don't see it being done along the Potomac shoreline. First, there are very few protected anchorages or coves, so you'd be mooring in an exposed stretch of the river. Yes, it's only a river, but below the bridges it's plenty wide enough that big chop develops. Also, the river is shoal in many places, so your options for moorage are severely limited by depth.
Finally, even if you could locate a suitable location, you would still need to find public access to get to your boat. With only very few exceptions, the shoreline is private property. The areas with public access already have marinas or don't allow boat launches.