I've observed a mechanic manually spinning the 412's tail rotor
I think you probably mean slowly turning which as has been pointed out is not specifically prohibited anywhere in the RFM or MM (that I have been able to find anyway!)
All the 412's that I have flown in the offshore role have been on the High Skid option and as a 6'2" midget - I find that I can't reach the bloody thing anyway so need steps to do a thorough pre flight (or rely on the much more highly qualified Engineer to carry that out and rely personally on a thorough visual inspection including checking for cracks i.a.w. the recent Airworthiness Directive)