In the US, is there a way for a pilot to legally "maneuver at will" in actual IMC?
You don't have to have permission to fly in IMC in uncontrolled, class G, airspace. However, you must have an instrument rating and be in an IFR-certified aircraft. In uncontrolled airspace, you may fly into IMC as you like and perform whatever maneuvers you want. It's a very good idea to get flight following in order to receive traffic warnings from ATC if you decide to fly within IMC in uncontrolled, class G, airspace. 

You do need to get permission to fly in controlled airspace (all classes except for G) if you are operating under IFR, regardless of actual meteorological conditions.  

Note: Classes A-D require permission under both IFR and VFR; classes E and G do not require permission under VFR.

If you want to practice in IMC within controlled airspace, you must explain your mission to ATC and obtain a clearance to operate under IFR. If ATC has the space and the time, they may give you a clearance for a specific area and perhaps a block altitude if you want to climb and descend. There aren't officially designated areas for practice, but if you ask local instructors, they will tell you where they usually go. Most of the time, the preference is to get out of controlled airspace if possible; it's just easier.