Is it possible to design a hardware wallet that can allow operation of a routing Lightning node with the security level of a hardware wallet?
Yes, a hardware wallet could be designed for use on the Lighting Network, but no, there's no such thing as eliminating risk of theft.  There is always risk of theft, no matter what you do.

A hardware wallet could be made to sign the transactions needed to construct the smart contracts for lightning channels, and could even be made to communicate with them.  This would keep the private keys out of the memory of the computer running the lightning node, thus reducing the risk...but not eliminating it.

There is always the chance that the protocol used to communicate to with the wallet is vulnerable to exploitation, or even the hardware wallet itself.  Even if the vulnerability didn't compromise the wallet, but merely rendered it inoperable for a time, an attack could be crafted where you end up forfeiting your money in the channel because you are unable to sign transactions.

Our technology is built on layers on top of layers on top of layers on top of layers.  Hardware wallets and tier 2 smart contract networks are no exception.  Anyone who tells you it is risk-free is selling something.