If you are on TDM 4.5, then you could use the From Occurrence and To Occurrence fields in FDM where you can specify a delimiter for a particular field.
So for e.g. to mask the first field in the delimited column you would use the masking rule and set the From Occurrence to Blank and To Occurrence to 1(|) and to mask the second field you would use 1(|) and 2(|) respectively.