Can Apple Watch unlock signal be used to unlock other TouchID-protected apps? I imagine this wouldn't be too tough to do with Automater, but I haven't found any obvious answers to this online thus far.

For example, let's say I use an app that requires me to authenticate with TouchID, or I'm making a payment online with ApplePay. I should be able to verify my identify with my watch as well (this is how ApplePay works in the "wild" anyway).

Would love to hear any ideas about how to do this and would be up for collaborating on a solution as well.
No, this currently isn't possible.  

We know Apple has the ability to use wifi/bluetooth to unlock your Mac with your Watch if you're close to it.  However, Apple has not made such a feature available to 3rd party developers as of this date.