Hello backslashV,
We appreciate your patience in waiting for a response on this post. We have done some research in regards to managing Windows Hello settings and external monitors and we are unable to locate a combination of any settings available to ensure that Windows Hello is disabled only when connected to an external monitor.
You could set up to have a PIN to be utilized as opposed to entering a password or having your face recognized every time the Surface Laptop requires a login. Or we could have a local account set up with no connected Microsoft account that doesn't require a password to log into the device. There are also fingerprint readers that would be compatible with your Surface Laptop.
*Please note: Finger Print readers may require your Surface Laptop to be running Windows 10 Pro to be able to install necessary drivers to function properly.
If you would like to discuss these account options in further detail, please feel free to respond to this post or contact our Surface Technical Support team via dialing 1-800-MICROSOFT or by going to our Support website and clicking Speak to a Live Person to schedule a callback.
Sincerely~
Mason L.
Microsoft Surface Support