Only if all the related elements are all virtual and on the blockchain can it do anything - so we're stuck with gambling and cryptokitties for real world usage.
Most investments regulated in the US cannot legally be bearer instruments. This means there must be a ledger listing who owns what with real names attached. For public companies they must use a registered transfer agent to keep these ownership records. Private companies can keep their own records, but in most cases cannot use bearer instruments. What happens if I transfer you the token if there is a master ownership ledger with names and addresses? Nothing happens. The token record becomes out of sync with reality is the only thing that happens.
Anything controlled by a private key is a bearer instrument by definition because the person bearing the private key has custody of the asset.
You're making up your own definition of transfer agent, not the one used by the SEC. They have specific requirements and responsibilities.
At best the key signing is providing a partial audit trail with no case law to support any liability transfer created by its use. Typically transfer agents take very little liability by requiring a medallion signature before allowing a transfer.
worried that loses the “trustless” of blockchain? hash the KYC entry and store it on chain so you make certain that the off-chain real world ownership information hasn’t been tampered with
What happens when someone loses their private keys? The asset by US law is tied to them as a person, not a bearer instrument like a private key.
An alternative might be found in protocols like UMA (https://umaproject.org/), Augur (https://www.augur.net/) or MakerDAO (https://makerdao.com/) which allow you create synthetic tokens that trustlessly track the price of a real-world asset by creating financial incentives to rebalance the price of the minted tokens. MakerDAO has already been used to create over $80MM in a synthetic USD token called Dai (https://makerdao.com/dai/) and UMA has created a synthetic S&P500 token, allowing anyone anywhere around the world to get exposure to US Stocks (https://medium.com/uma-project/announcing-us-stock-index-tok...).