In the [qgisfunctison.py](https://github.com/DMS-Aus/Roam/blob/8fd39f90e7890f73a26bd28e421a73566fa0e02b/src/roam/qgisfunctions.py#L74) add a new function with the following

```
@qgsfunction(0, 'z')
def _z(values, feature, parent):
    if hasattr(capturegeometry, 'z'):
          return capturegeometry.z
    else:
          return None
```

This will allow a field to have the default `[% $z %]` now.