HC - my answer would be that you can't because there is no specific clearance to say you can. Since every pilot from day one is taught the dangers of freezing rain and drizzle it seems reasonable to argue that therefore being able to fly in it would require some special equipment and/or clearance. You can anti-ice your engines and you can de-ice your MR and TR blades (with the right fit) but to my knowledge there is no method of de-icing either the airframe or the pitch change mechanisms of the rotors, both of which will be badly affected by freezing precipitation.
I have only encountered freezing rain once and then only briefly but it was more than enough to convince me that all the warnings are valid.
It is always interesting to see which pilots go for 'I can do it because it doesn't say I can't' versus 'I can't do it because it doesn't say I can' when quoting rules and regulations.