Certainly not hard... it's all in the night-vision equipment... almost the same way that the follow each other around in the Unimogs (trucks) at night with no lights on! They also use ultraviolet lamps on trucks that are more easily visible through night vision equipment. Similar setup in the aircraft, and in IMC they actually move into a CLOSER formation than in VFR! You'd be surprised how far you can actually see when you're in cloud, you can normally see your wingtips, so you can easily see another aircraft, but believe me they do get lost from the formation occasionally and forget re-joining... the guy that drops off has to get traffic updates from the friendly AirServices operator so that he doesn't smack up their clackers - I have heard this on VHF several times.
Harvard in IMC & aero's --- Not legal.
Roulettes in IMC formation - yep sure...
Ever notice how the roulettes are exempt from the standard airshow rules and can fly towards and over the crowd at any time...? They are seemingly above the law, but in my opinion not beyond crashing into the crowd; really not sure how the rules don't apply to them.