Thanks dop2000, that does work
For anyone else having trouble, it seems that the object only gets added to the Family immediately when you do a Pick Last Created on that family.
So:
Create object by name, "House"
Pick Last SelectableObjects
Works as expected, SelectableObjects.ObjectTypeName = "House"
But:
Pick Buildings where Buildings.UID = SelectableObjects.UID
Doesn't work, nothing is picked
This does work tho:
Pick Last Buildings
if Buildings.UID = SelectableObjects.UID Browser.Log Buildings.ObjectTypeName
Logs "House" in the Browser
Hope that helps anyone as confused as me!