regarding your question: Something like that doesn't work in Collect. You can try if you fork on your own fork but why? Can't you use default values? What is your scenario?
I am involved in an application that needs a login screen. Within the application, we implement several ODK forms and in those forms we use the "username" metadata to deploy custom data.
so... We need the login screen to change these metadata.
We accomplished this by changing the source code, but we wanted to know if there is a way to make this change without touching the code