Coming back to this, I figured out how to set default values without requiring any other changes. In typsy v0.2.0, the following will work: tree-counter(()=>{}, default: 3).
Explaining a couple of the things you can see here: tree-counter is the main counter routine in typsy, so named because it supports creating ‘subcounters’, e.g. to number theorems according to the section that they are in. The ()=>{} is a trick to obtain a unique counter each time (without having to find a unique string), by taking advantage of the fact that all anonymous functions have different identities.