Metform 215 gave the freezing level before the front as 0', and after the front as 6000', so we figured that the actual freezing level was probably somewhere between the two.
It doesn't tend to work like that with fronts. The warm air behind is warm and the cold air in front is cold. If you are in the cold sector the freezing level will be as per the cold sector. What you encountered was rain from the warm sector falling into the cold sector and becoming super cooled (or that's what it sounds like anyway). I was flying in the vicinity of the front today and that type of icing was a distinct possibility but I did not encounter any.
Personally I never fly (without de-ice) in freezing conditions where ice is likely unless the temp at MSA will allow removal of said ice, or I know I can become VMC in order to descend further to a positive temp (ie base higher than MSA).
Sounds like you did your homework, got caught out a bit, and have learned from it. The airmet text often gives info on sub zero layers by the way. I think the balooning forecast does also.