Hi there.
ICAO DOC 4444 states, under 5.9,  that " When so requested by an aircraft and provided it is agreed by the pilot of the other aircraft and so authorized by the appropriate ATS authority, an ATC unit may clear a controlled flight, including departing and arriving flights, operating in airspace Classes D and E in visual meteorological conditions during the hours of daylight to fly subject to maintaining own separation to one other aircraft and remaining in visual meteorological conditions."

So this is not possible in IMC. Furthermore, it seems that it is not possible in any airspace Classes other than D and E, even in VMC.