Hi Ward,
This functionality for offline profiles is yet not available out of the box in Mendix.
You can accomplish this right now with some custom JavaScript. Here's how you would do it:
this.contextObj.set("AttributeName","NewAttributeValue");
I ran into the same problem as you and I solved it by creating a custom widget where I could select in the modeler which attribute I want to pass and what the default value I want it to be.
Thank you both for your answers!
For a quick fix we now used the Mendix Switch widget (which is always default on false) and made it invisible on the page.
Will definitely try your suggestion :) !