For an sns to be able to set the ledger parameters: transfer_fee, token_symbol, token_decimals, etc, the sns-governance canister needs a new proposal action type that can upgrade the ledger with the new parameters. The ledger already supports upgrading most parameters through the post_upgrade arg on the ledger, so the sns-governance can use that to set the parameters. Here is the code: https://github.com/levifeldman/ic/pull/1/files.
Proposal post for this sns capability: