LIfe jackets are approved under
TSO-C 13f, as per
CAO 103.13 (NB the CAO mentions the superceeded TSO-C 13e), or
ATSO-1C13
Each manufacturer details their own maintenance standard for each type of life jacket, which includes the method of storage. This means you will need to write to the individual jacket manufacturer and ask if their required maintenance schedule includes the jacket being sealed within the plastic sheath.
Reading the TSO, the standard requires the manufacturer to guarantee the seals and material for the inspection period of the life preserver (along with the method of storage), and you can see that the "cheapies" used by the airlines aren't particularly tough. I would assume that the tough plastic sheath is required for storage, in order to protect the seal and material integrity. Life preservers that I have seen and used that weren't stored in a plastic sheath were all built from very tough nylon, much tougher than the stored types.
I think that you will be out of luck, should the jacket not be stored in the plastic sheath provided.