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.
aviation.stackexchange.com/users/1791/GdD
Yes.
You can requests a local IFR, or quadrant clearance and then receive a clearance for a region, such as: CLEARED TO FLY (general direction from NAVAID)
OF (NAVAID name and type) BETWEEN (specified) COURSES TO/BEARINGS
FROM/RADIALS (NAVAID name when a NDB)
WITHIN (number of miles)
MILE RADIUS, And an example:
“Cleared to fly east of Allentown VORTAC between the zero four five and the one three five radials within four zero mile radius.”
Both are from FAA Order JO 7110.65W 4-1-1
aviation.stackexchange.com/users/4405/Adam
"Maneuvering at will" within a particular area is called "work".
This is particularly common for news/police helicopters, which often need to wander around above some event in class B/C airspace.
I've only heard it done as VFR, but I don't see a reason why ATC wouldn't allow it for IFR if there's nobody else around.
Be clear about the horizontal and vertical boundaries you want in your request, and let them know when you're done working and want to resume normal control.
You may want to call the relevant ATC unit ahead of time to see when/where will give you the best chance of approval and what exactly to put on your flight plan since this is obviously not a standard request.
aviation.stackexchange.com/users/35166/StephenS
You might have better luck working with a controller at a Class D tower near you, or maybe Class C. Find an intersection or something identifiable from an approach plate that's in their airspace, request the maneuvers you want.
They can have you report in as you maneuver, and yet still have control if they need room for some one actually landing, or keep you clear of transiting traffice, etc.
aviation.stackexchange.com/users/28945/CrossRoads
I have done this while operating under IFR in IMC several times.
Just ask for a block airspace and tell them what you want.
I've found ATC to be very accommodating.
aviation.stackexchange.com/users/8531/Pugz
In the US, is there a way for a pilot to legally "maneuver at will" in actual IMC?   Let's say you want to practice a series of reversing 360-degree turns and not have to communicate every change in heading to a controller or follow a controller's step-by-step instructions.  For example, is there such a thing as an IMC "practice area" where only one aircraft at a time is allowed to operate under IFR?  Also, is there a distinction between controlled and uncontrolled airspace in this regard?
In the US, is there a way for a pilot to legally "maneuver at will" in actual IMC?
aviation.stackexchange.com
<faa-regulations><flight-training><instrument-flight-rules><instrument-procedures>
aviation.stackexchange.com/questions/56680
aviation.stackexchange.com/users/34686/quiet flyer
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. However, you must have an instrument rating and be in an IFR-certified aircraft. You might have better luck working with a controller at a Class D tower near you, or maybe Class C. Find an intersection or something identifiable from an approach plate that's in their airspace, request the maneuvers you want.  In order to maneuver in an actual IMR, a pilot should not have permission as far as he is in an uncontrolled airspace. Another answer claims that an instrument rating and a certified aircraft is needed. Finally, the pilot should contact ATC to give the details of his flight and ask for permission or should contact a local IFR and get clearance or  should contact a controller of a class D tower, closeby. 
A pilot can maneuver at will without permission in uncontrolled airspace.  An instrument rating and a certified aircraft is needed.  You should contact ATC to give the details of your flight and ask for permission.  You should contact a local IFR and get clearance. You should contact a controller of a class D tower closeby. 