Hi Remnerud,
From your description, I understand that when creating a project site, you need to be able to fill in the default value for fields in site library, then the fields default value will be used on the site library.
If so, the considerable way I can suggest is using a web part on the site, or update the site create page.
The first way is to add a web part on the project site template home page. In the web part, it will pop up a model dialog, to fill in the fields value, to make sure the dialog just opens when first accessing the site, we can store the fields variable value
in a list on the project, then check whether the fields values are existing in the list, if so, dont pop up the dialog, if not, pop up the dialog to fill in the information.
- Custom a project site template, include a list instance to store the filled in the fields default values.
- Create a web part, add it the home page of the project site template.
- In the web part, first check whether the list contains the filled fields value, if not, pop up a modal dialog to fill in the information.
For the second workaround, custom the site created page, which in my mind is not so recommended, in SharePoint 2013, we use _layouts/15/newsbweb.aspx to create a site, to pop up a page to fill in the fields default value, we need to add script to save button
of the site creation page, to pop up a dialog if project site template is selected on the page, then fill in the information, store it on the site first, use site created method to set it to default value of the fields in library, however, this update to the
application page will be used on the whole farm.
Thanks,