With the Secure Shell Chrome app I can provide a SSH server relay options of something like:
--ssh-agent=hutaonesu (ID of app providing the agent). This allows me to use a Yubikey for hardware-backed SSH key.

I log in with hardware-protected keys on all my machines currently, and would like to switch from Secure Shell on ChromeOS to Mosh (for obvious reasons), but losing HW key support is a deal breaker for me.

Note that there's more than one app that can provide SSH agent support, so I think doing the same as Secure Shell, allowing user to specify, is probably best.